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一、简便计算下面各题。
$\frac {1}{3}÷\frac {5}{8}+\frac {2}{3}÷\frac {5}{8}$
$\frac {11}{7}-\frac {8}{21}×\frac {5}{8}-\frac {4}{7}$
$\frac {2}{3}+\frac {4}{5}+\frac {4}{3}+\frac {1}{5}$
$30\% ×\frac {4}{9}-\frac {1}{9}×30\% $
$(\frac {5}{6}-\frac {2}{9})×18$
$999\frac {8}{9}+99\frac {8}{9}+9\frac {8}{9}$
$(\frac {3}{16}+\frac {1}{24})×8+\frac {2}{3}$
$(\frac {11}{12}+\frac {7}{18}+\frac {5}{24})×72$
$\frac {3}{4}×\frac {5}{6}+\frac {5}{6}÷4$
$\frac {1}{5}×37\% +80\% ×37\% $
$\frac {7}{4}-\frac {5}{16}×\frac {4}{3}-\frac {7}{12}$
$\frac {4}{5}÷\frac {5}{6}÷\frac {6}{7}÷87.5\% $
$\frac {1}{3}÷\frac {5}{8}+\frac {2}{3}÷\frac {5}{8}$
$\frac {11}{7}-\frac {8}{21}×\frac {5}{8}-\frac {4}{7}$
$\frac {2}{3}+\frac {4}{5}+\frac {4}{3}+\frac {1}{5}$
$30\% ×\frac {4}{9}-\frac {1}{9}×30\% $
$(\frac {5}{6}-\frac {2}{9})×18$
$999\frac {8}{9}+99\frac {8}{9}+9\frac {8}{9}$
$(\frac {3}{16}+\frac {1}{24})×8+\frac {2}{3}$
$(\frac {11}{12}+\frac {7}{18}+\frac {5}{24})×72$
$\frac {3}{4}×\frac {5}{6}+\frac {5}{6}÷4$
$\frac {1}{5}×37\% +80\% ×37\% $
$\frac {7}{4}-\frac {5}{16}×\frac {4}{3}-\frac {7}{12}$
$\frac {4}{5}÷\frac {5}{6}÷\frac {6}{7}÷87.5\% $
答案
一、$\frac{8}{5}$ $\frac{16}{21}$ 3 $\frac{1}{10}$ 11 $1109\frac{2}{3}$ $2\frac{1}{2}$ 109 $\frac{5}{6}$ 0.37 $\frac{3}{4}$ $\frac{32}{25}$
解析
1. $\frac{1}{3}÷\frac{5}{8}+\frac{2}{3}÷\frac{5}{8}$
$=(\frac{1}{3}+\frac{2}{3})×\frac{8}{5}$
$=1×\frac{8}{5}$
$=\frac{8}{5}$
2. $\frac{11}{7}-\frac{8}{21}×\frac{5}{8}-\frac{4}{7}$
$=\frac{11}{7}-\frac{5}{21}-\frac{4}{7}$
$=(\frac{11}{7}-\frac{4}{7})-\frac{5}{21}$
$=1-\frac{5}{21}$
$=\frac{16}{21}$
3. $\frac{2}{3}+\frac{4}{5}+\frac{4}{3}+\frac{1}{5}$
$=(\frac{2}{3}+\frac{4}{3})+(\frac{4}{5}+\frac{1}{5})$
$=2+1$
$=3$
4. $30\% ×\frac{4}{9}-\frac{1}{9}×30\%$
$=30\%×(\frac{4}{9}-\frac{1}{9})$
$=\frac{3}{10}×\frac{1}{3}$
$=\frac{1}{10}$
5. $(\frac{5}{6}-\frac{2}{9})×18$
$=\frac{5}{6}×18-\frac{2}{9}×18$
$=15 - 4$
$=11$
6. $999\frac{8}{9}+99\frac{8}{9}+9\frac{8}{9}$
$=(1000 - \frac{1}{9}) + (100 - \frac{1}{9}) + (10 - \frac{1}{9})$
$=1000 + 100 + 10 - (\frac{1}{9}+\frac{1}{9}+\frac{1}{9})$
$=1110 - \frac{1}{3}$
$=1109\frac{2}{3}$
7. $(\frac{3}{16}+\frac{1}{24})×8+\frac{2}{3}$
$=\frac{3}{16}×8 + \frac{1}{24}×8 + \frac{2}{3}$
$=\frac{3}{2} + \frac{1}{3} + \frac{2}{3}$
$=\frac{3}{2} + 1$
$=2\frac{1}{2}$
8. $(\frac{11}{12}+\frac{7}{18}+\frac{5}{24})×72$
$=\frac{11}{12}×72 + \frac{7}{18}×72 + \frac{5}{24}×72$
$=66 + 28 + 15$
$=109$
9. $\frac{3}{4}×\frac{5}{6}+\frac{5}{6}÷4$
$=\frac{3}{4}×\frac{5}{6}+\frac{5}{6}×\frac{1}{4}$
$=\frac{5}{6}×(\frac{3}{4}+\frac{1}{4})$
$=\frac{5}{6}×1$
$=\frac{5}{6}$
10. $\frac{1}{5}×37\% +80\% ×37\%$
$=37\%×(\frac{1}{5}+0.8)$
$=0.37×(0.2 + 0.8)$
$=0.37×1$
$=0.37$
11. $\frac{7}{4}-\frac{5}{16}×\frac{4}{3}-\frac{7}{12}$
$=\frac{7}{4}-\frac{5}{12}-\frac{7}{12}$
$=\frac{7}{4}-(\frac{5}{12}+\frac{7}{12})$
$=\frac{7}{4}-1$
$=\frac{3}{4}$
12. $\frac{4}{5}÷\frac{5}{6}÷\frac{6}{7}÷87.5\%$
$=\frac{4}{5}×\frac{6}{5}×\frac{7}{6}×\frac{8}{7}$
$=\frac{4}{5}×\frac{8}{5}$
$=\frac{32}{25}$
$=(\frac{1}{3}+\frac{2}{3})×\frac{8}{5}$
$=1×\frac{8}{5}$
$=\frac{8}{5}$
2. $\frac{11}{7}-\frac{8}{21}×\frac{5}{8}-\frac{4}{7}$
$=\frac{11}{7}-\frac{5}{21}-\frac{4}{7}$
$=(\frac{11}{7}-\frac{4}{7})-\frac{5}{21}$
$=1-\frac{5}{21}$
$=\frac{16}{21}$
3. $\frac{2}{3}+\frac{4}{5}+\frac{4}{3}+\frac{1}{5}$
$=(\frac{2}{3}+\frac{4}{3})+(\frac{4}{5}+\frac{1}{5})$
$=2+1$
$=3$
4. $30\% ×\frac{4}{9}-\frac{1}{9}×30\%$
$=30\%×(\frac{4}{9}-\frac{1}{9})$
$=\frac{3}{10}×\frac{1}{3}$
$=\frac{1}{10}$
5. $(\frac{5}{6}-\frac{2}{9})×18$
$=\frac{5}{6}×18-\frac{2}{9}×18$
$=15 - 4$
$=11$
6. $999\frac{8}{9}+99\frac{8}{9}+9\frac{8}{9}$
$=(1000 - \frac{1}{9}) + (100 - \frac{1}{9}) + (10 - \frac{1}{9})$
$=1000 + 100 + 10 - (\frac{1}{9}+\frac{1}{9}+\frac{1}{9})$
$=1110 - \frac{1}{3}$
$=1109\frac{2}{3}$
7. $(\frac{3}{16}+\frac{1}{24})×8+\frac{2}{3}$
$=\frac{3}{16}×8 + \frac{1}{24}×8 + \frac{2}{3}$
$=\frac{3}{2} + \frac{1}{3} + \frac{2}{3}$
$=\frac{3}{2} + 1$
$=2\frac{1}{2}$
8. $(\frac{11}{12}+\frac{7}{18}+\frac{5}{24})×72$
$=\frac{11}{12}×72 + \frac{7}{18}×72 + \frac{5}{24}×72$
$=66 + 28 + 15$
$=109$
9. $\frac{3}{4}×\frac{5}{6}+\frac{5}{6}÷4$
$=\frac{3}{4}×\frac{5}{6}+\frac{5}{6}×\frac{1}{4}$
$=\frac{5}{6}×(\frac{3}{4}+\frac{1}{4})$
$=\frac{5}{6}×1$
$=\frac{5}{6}$
10. $\frac{1}{5}×37\% +80\% ×37\%$
$=37\%×(\frac{1}{5}+0.8)$
$=0.37×(0.2 + 0.8)$
$=0.37×1$
$=0.37$
11. $\frac{7}{4}-\frac{5}{16}×\frac{4}{3}-\frac{7}{12}$
$=\frac{7}{4}-\frac{5}{12}-\frac{7}{12}$
$=\frac{7}{4}-(\frac{5}{12}+\frac{7}{12})$
$=\frac{7}{4}-1$
$=\frac{3}{4}$
12. $\frac{4}{5}÷\frac{5}{6}÷\frac{6}{7}÷87.5\%$
$=\frac{4}{5}×\frac{6}{5}×\frac{7}{6}×\frac{8}{7}$
$=\frac{4}{5}×\frac{8}{5}$
$=\frac{32}{25}$
二、计算下面各题,怎样简便就怎样计算。
$(\frac {2}{3}+\frac {6}{7}×\frac {7}{24})÷\frac {11}{32}$
$(\frac {1}{2}+\frac {1}{3})÷\frac {3}{5}-\frac {17}{18}$
$\frac {2}{5}×\frac {5}{7}+\frac {2}{7}$
$\frac {5}{6}×(\frac {2}{9}÷\frac {5}{6}+\frac {5}{6})$
$21÷(\frac {1}{3}+\frac {2}{5})÷\frac {9}{11}$
$(\frac {19}{20}-\frac {1}{4})÷\frac {5}{12}$
$\frac {12}{5}×(\frac {5}{6}+\frac {3}{4})+80\% $
$(\frac {7}{9}-\frac {5}{18})×(\frac {3}{4}+\frac {5}{8})$
$\frac {1}{8}÷(1-50\% -\frac {1}{4})$
$(\frac {5}{6}+\frac {7}{12}+\frac {3}{8})÷\frac {1}{48}$
$\frac {1}{2}+(\frac {3}{4}-\frac {1}{12})×\frac {3}{5}$
$\frac {7}{8}×80\% +20\% ×\frac {1}{8}$
$\frac {1}{3}÷(\frac {2}{3}-\frac {2}{5})×\frac {3}{5}$
$\frac {4}{5}×(\frac {5}{6}+\frac {3}{4})+\frac {4}{5}$
$(\frac {7}{8}-\frac {1}{2})÷(\frac {1}{2}÷4)$
$\frac {6}{7}×40\% +\frac {1}{7}×40\% $
$\frac {8}{9}×[\frac {3}{7}÷(\frac {2}{3}+\frac {1}{7})]$
$\frac {1}{5}-(\frac {1}{5}×\frac {1}{3}-\frac {1}{30})$
$(\frac {2}{3}+\frac {6}{7}×\frac {7}{24})÷\frac {11}{32}$
$(\frac {1}{2}+\frac {1}{3})÷\frac {3}{5}-\frac {17}{18}$
$\frac {2}{5}×\frac {5}{7}+\frac {2}{7}$
$\frac {5}{6}×(\frac {2}{9}÷\frac {5}{6}+\frac {5}{6})$
$21÷(\frac {1}{3}+\frac {2}{5})÷\frac {9}{11}$
$(\frac {19}{20}-\frac {1}{4})÷\frac {5}{12}$
$\frac {12}{5}×(\frac {5}{6}+\frac {3}{4})+80\% $
$(\frac {7}{9}-\frac {5}{18})×(\frac {3}{4}+\frac {5}{8})$
$\frac {1}{8}÷(1-50\% -\frac {1}{4})$
$(\frac {5}{6}+\frac {7}{12}+\frac {3}{8})÷\frac {1}{48}$
$\frac {1}{2}+(\frac {3}{4}-\frac {1}{12})×\frac {3}{5}$
$\frac {7}{8}×80\% +20\% ×\frac {1}{8}$
$\frac {1}{3}÷(\frac {2}{3}-\frac {2}{5})×\frac {3}{5}$
$\frac {4}{5}×(\frac {5}{6}+\frac {3}{4})+\frac {4}{5}$
$(\frac {7}{8}-\frac {1}{2})÷(\frac {1}{2}÷4)$
$\frac {6}{7}×40\% +\frac {1}{7}×40\% $
$\frac {8}{9}×[\frac {3}{7}÷(\frac {2}{3}+\frac {1}{7})]$
$\frac {1}{5}-(\frac {1}{5}×\frac {1}{3}-\frac {1}{30})$
答案
二、$\frac{8}{3}$ $\frac{4}{9}$ $\frac{4}{7}$ $\frac{11}{12}$ 35 $\frac{42}{25}$ $\frac{23}{5}$ $\frac{11}{16}$ $\frac{1}{2}$ 86 $\frac{9}{10}$ $\frac{29}{40}$ $\frac{3}{4}$ $\frac{31}{15}$ 3 0.4 $\frac{8}{17}$ $\frac{1}{6}$
解析
1. $(\frac{2}{3}+\frac{6}{7}×\frac{7}{24})÷\frac{11}{32}$
$=(\frac{2}{3}+\frac{1}{4})×\frac{32}{11}$
$=(\frac{8}{12}+\frac{3}{12})×\frac{32}{11}$
$=\frac{11}{12}×\frac{32}{11}$
$=\frac{8}{3}$
2. $(\frac{1}{2}+\frac{1}{3})÷\frac{3}{5}-\frac{17}{18}$
$=\frac{5}{6}×\frac{5}{3}-\frac{17}{18}$
$=\frac{25}{18}-\frac{17}{18}$
$=\frac{8}{18}$
$=\frac{4}{9}$
3. $\frac{2}{5}×\frac{5}{7}+\frac{2}{7}$
$=\frac{2}{7}+\frac{2}{7}$
$=\frac{4}{7}$
4. $\frac{5}{6}×(\frac{2}{9}÷\frac{5}{6}+\frac{5}{6})$
$=\frac{5}{6}×(\frac{2}{9}×\frac{6}{5}+\frac{5}{6})$
$=\frac{5}{6}×(\frac{4}{15}+\frac{5}{6})$
$=\frac{5}{6}×(\frac{8}{30}+\frac{25}{30})$
$=\frac{5}{6}×\frac{33}{30}$
$=\frac{11}{12}$
5. $21÷(\frac{1}{3}+\frac{2}{5})÷\frac{9}{11}$
$=21÷\frac{11}{15}×\frac{11}{9}$
$=21×\frac{15}{11}×\frac{11}{9}$
$=21×\frac{15}{9}$
$=35$
6. $(\frac{19}{20}-\frac{1}{4})÷\frac{5}{12}$
$=(\frac{19}{20}-\frac{5}{20})×\frac{12}{5}$
$=\frac{14}{20}×\frac{12}{5}$
$=\frac{42}{25}$
7. $\frac{12}{5}×(\frac{5}{6}+\frac{3}{4})+80\%$
$=\frac{12}{5}×(\frac{10}{12}+\frac{9}{12})+\frac{4}{5}$
$=\frac{12}{5}×\frac{19}{12}+\frac{4}{5}$
$=\frac{19}{5}+\frac{4}{5}$
$=\frac{23}{5}$
8. $(\frac{7}{9}-\frac{5}{18})×(\frac{3}{4}+\frac{5}{8})$
$=(\frac{14}{18}-\frac{5}{18})×(\frac{6}{8}+\frac{5}{8})$
$=\frac{9}{18}×\frac{11}{8}$
$=\frac{1}{2}×\frac{11}{8}$
$=\frac{11}{16}$
9. $\frac{1}{8}÷(1-50\%-\frac{1}{4})$
$=\frac{1}{8}÷(\frac{1}{2}-\frac{1}{4})$
$=\frac{1}{8}÷\frac{1}{4}$
$=\frac{1}{8}×4$
$=\frac{1}{2}$
10. $(\frac{5}{6}+\frac{7}{12}+\frac{3}{8})÷\frac{1}{48}$
$=(\frac{5}{6}+\frac{7}{12}+\frac{3}{8})×48$
$=\frac{5}{6}×48+\frac{7}{12}×48+\frac{3}{8}×48$
$=40 + 28 + 18$
$=86$
11. $\frac{1}{2}+(\frac{3}{4}-\frac{1}{12})×\frac{3}{5}$
$=\frac{1}{2}+(\frac{9}{12}-\frac{1}{12})×\frac{3}{5}$
$=\frac{1}{2}+\frac{8}{12}×\frac{3}{5}$
$=\frac{1}{2}+\frac{2}{3}×\frac{3}{5}$
$=\frac{1}{2}+\frac{2}{5}$
$=\frac{5}{10}+\frac{4}{10}$
$=\frac{9}{10}$
12. $\frac{7}{8}×80\% + 20\%×\frac{1}{8}$
$=\frac{7}{8}×\frac{4}{5}+\frac{1}{5}×\frac{1}{8}$
$=\frac{7}{10}+\frac{1}{40}$
$=\frac{28}{40}+\frac{1}{40}$
$=\frac{29}{40}$
13. $\frac{1}{3}÷(\frac{2}{3}-\frac{2}{5})×\frac{3}{5}$
$=\frac{1}{3}÷(\frac{10}{15}-\frac{6}{15})×\frac{3}{5}$
$=\frac{1}{3}÷\frac{4}{15}×\frac{3}{5}$
$=\frac{1}{3}×\frac{15}{4}×\frac{3}{5}$
$=\frac{1}{3}×\frac{9}{4}$
$=\frac{3}{4}$
14. $\frac{4}{5}×(\frac{5}{6}+\frac{3}{4})+\frac{4}{5}$
$=\frac{4}{5}×(\frac{10}{12}+\frac{9}{12})+\frac{4}{5}$
$=\frac{4}{5}×\frac{19}{12}+\frac{4}{5}$
$=\frac{19}{15}+\frac{12}{15}$
$=\frac{31}{15}$
15. $(\frac{7}{8}-\frac{1}{2})÷(\frac{1}{2}÷4)$
$=(\frac{7}{8}-\frac{4}{8})÷(\frac{1}{2}×\frac{1}{4})$
$=\frac{3}{8}÷\frac{1}{8}$
$=3$
16. $\frac{6}{7}×40\% +\frac{1}{7}×40\%$
$=(\frac{6}{7}+\frac{1}{7})×0.4$
$=1×0.4$
$=0.4$
17. $\frac{8}{9}×[\frac{3}{7}÷(\frac{2}{3}+\frac{1}{7})]$
$=\frac{8}{9}×[\frac{3}{7}÷(\frac{14}{21}+\frac{3}{21})]$
$=\frac{8}{9}×[\frac{3}{7}÷\frac{17}{21}]$
$=\frac{8}{9}×[\frac{3}{7}×\frac{21}{17}]$
$=\frac{8}{9}×\frac{9}{17}$
$=\frac{8}{17}$
18. $\frac{1}{5}-(\frac{1}{5}×\frac{1}{3}-\frac{1}{30})$
$=\frac{1}{5}-(\frac{1}{15}-\frac{1}{30})$
$=\frac{1}{5}-(\frac{2}{30}-\frac{1}{30})$
$=\frac{1}{5}-\frac{1}{30}$
$=\frac{6}{30}-\frac{1}{30}$
$=\frac{5}{30}$
$=\frac{1}{6}$
$=(\frac{2}{3}+\frac{1}{4})×\frac{32}{11}$
$=(\frac{8}{12}+\frac{3}{12})×\frac{32}{11}$
$=\frac{11}{12}×\frac{32}{11}$
$=\frac{8}{3}$
2. $(\frac{1}{2}+\frac{1}{3})÷\frac{3}{5}-\frac{17}{18}$
$=\frac{5}{6}×\frac{5}{3}-\frac{17}{18}$
$=\frac{25}{18}-\frac{17}{18}$
$=\frac{8}{18}$
$=\frac{4}{9}$
3. $\frac{2}{5}×\frac{5}{7}+\frac{2}{7}$
$=\frac{2}{7}+\frac{2}{7}$
$=\frac{4}{7}$
4. $\frac{5}{6}×(\frac{2}{9}÷\frac{5}{6}+\frac{5}{6})$
$=\frac{5}{6}×(\frac{2}{9}×\frac{6}{5}+\frac{5}{6})$
$=\frac{5}{6}×(\frac{4}{15}+\frac{5}{6})$
$=\frac{5}{6}×(\frac{8}{30}+\frac{25}{30})$
$=\frac{5}{6}×\frac{33}{30}$
$=\frac{11}{12}$
5. $21÷(\frac{1}{3}+\frac{2}{5})÷\frac{9}{11}$
$=21÷\frac{11}{15}×\frac{11}{9}$
$=21×\frac{15}{11}×\frac{11}{9}$
$=21×\frac{15}{9}$
$=35$
6. $(\frac{19}{20}-\frac{1}{4})÷\frac{5}{12}$
$=(\frac{19}{20}-\frac{5}{20})×\frac{12}{5}$
$=\frac{14}{20}×\frac{12}{5}$
$=\frac{42}{25}$
7. $\frac{12}{5}×(\frac{5}{6}+\frac{3}{4})+80\%$
$=\frac{12}{5}×(\frac{10}{12}+\frac{9}{12})+\frac{4}{5}$
$=\frac{12}{5}×\frac{19}{12}+\frac{4}{5}$
$=\frac{19}{5}+\frac{4}{5}$
$=\frac{23}{5}$
8. $(\frac{7}{9}-\frac{5}{18})×(\frac{3}{4}+\frac{5}{8})$
$=(\frac{14}{18}-\frac{5}{18})×(\frac{6}{8}+\frac{5}{8})$
$=\frac{9}{18}×\frac{11}{8}$
$=\frac{1}{2}×\frac{11}{8}$
$=\frac{11}{16}$
9. $\frac{1}{8}÷(1-50\%-\frac{1}{4})$
$=\frac{1}{8}÷(\frac{1}{2}-\frac{1}{4})$
$=\frac{1}{8}÷\frac{1}{4}$
$=\frac{1}{8}×4$
$=\frac{1}{2}$
10. $(\frac{5}{6}+\frac{7}{12}+\frac{3}{8})÷\frac{1}{48}$
$=(\frac{5}{6}+\frac{7}{12}+\frac{3}{8})×48$
$=\frac{5}{6}×48+\frac{7}{12}×48+\frac{3}{8}×48$
$=40 + 28 + 18$
$=86$
11. $\frac{1}{2}+(\frac{3}{4}-\frac{1}{12})×\frac{3}{5}$
$=\frac{1}{2}+(\frac{9}{12}-\frac{1}{12})×\frac{3}{5}$
$=\frac{1}{2}+\frac{8}{12}×\frac{3}{5}$
$=\frac{1}{2}+\frac{2}{3}×\frac{3}{5}$
$=\frac{1}{2}+\frac{2}{5}$
$=\frac{5}{10}+\frac{4}{10}$
$=\frac{9}{10}$
12. $\frac{7}{8}×80\% + 20\%×\frac{1}{8}$
$=\frac{7}{8}×\frac{4}{5}+\frac{1}{5}×\frac{1}{8}$
$=\frac{7}{10}+\frac{1}{40}$
$=\frac{28}{40}+\frac{1}{40}$
$=\frac{29}{40}$
13. $\frac{1}{3}÷(\frac{2}{3}-\frac{2}{5})×\frac{3}{5}$
$=\frac{1}{3}÷(\frac{10}{15}-\frac{6}{15})×\frac{3}{5}$
$=\frac{1}{3}÷\frac{4}{15}×\frac{3}{5}$
$=\frac{1}{3}×\frac{15}{4}×\frac{3}{5}$
$=\frac{1}{3}×\frac{9}{4}$
$=\frac{3}{4}$
14. $\frac{4}{5}×(\frac{5}{6}+\frac{3}{4})+\frac{4}{5}$
$=\frac{4}{5}×(\frac{10}{12}+\frac{9}{12})+\frac{4}{5}$
$=\frac{4}{5}×\frac{19}{12}+\frac{4}{5}$
$=\frac{19}{15}+\frac{12}{15}$
$=\frac{31}{15}$
15. $(\frac{7}{8}-\frac{1}{2})÷(\frac{1}{2}÷4)$
$=(\frac{7}{8}-\frac{4}{8})÷(\frac{1}{2}×\frac{1}{4})$
$=\frac{3}{8}÷\frac{1}{8}$
$=3$
16. $\frac{6}{7}×40\% +\frac{1}{7}×40\%$
$=(\frac{6}{7}+\frac{1}{7})×0.4$
$=1×0.4$
$=0.4$
17. $\frac{8}{9}×[\frac{3}{7}÷(\frac{2}{3}+\frac{1}{7})]$
$=\frac{8}{9}×[\frac{3}{7}÷(\frac{14}{21}+\frac{3}{21})]$
$=\frac{8}{9}×[\frac{3}{7}÷\frac{17}{21}]$
$=\frac{8}{9}×[\frac{3}{7}×\frac{21}{17}]$
$=\frac{8}{9}×\frac{9}{17}$
$=\frac{8}{17}$
18. $\frac{1}{5}-(\frac{1}{5}×\frac{1}{3}-\frac{1}{30})$
$=\frac{1}{5}-(\frac{1}{15}-\frac{1}{30})$
$=\frac{1}{5}-(\frac{2}{30}-\frac{1}{30})$
$=\frac{1}{5}-\frac{1}{30}$
$=\frac{6}{30}-\frac{1}{30}$
$=\frac{5}{30}$
$=\frac{1}{6}$
