一、口算。
$3-\frac {5}{4}=$
$\frac {21}{20}÷\frac {3}{5}=$
$\frac {3}{5}-\frac {7}{20}=$
$\frac {4}{9}×\frac {1}{8}=$
$\frac {33}{35}×\frac {5}{11}=$
$\frac {1}{4}÷\frac {5}{24}=$
$\frac {1}{2}+\frac {3}{5}=$
$\frac {3}{7}÷21=$
$\frac {3}{4}×\frac {10}{21}=$
$10×\frac {1}{5}=$
$\frac {2}{3}×\frac {5}{12}=$
$\frac {7}{16}+\frac {1}{8}=$
$\frac {3}{4}-\frac {1}{2}=$
$\frac {4}{15}÷\frac {1}{3}=$
$\frac {1}{3}×\frac {6}{5}=$
$4÷\frac {8}{21}=$
$\frac {3}{7}-\frac {1}{6}=$
$\frac {3}{10}÷\frac {7}{15}=$
$\frac {4}{9}×\frac {3}{20}=$
$\frac {9}{16}÷\frac {3}{4}=$
$\frac {3}{5}×\frac {10}{3}=$

一、$1\frac{3}{4}$ $\frac{7}{4}$ $\frac{1}{4}$ $\frac{1}{18}$ $\frac{3}{7}$ $\frac{6}{5}$ $\frac{11}{10}$ $\frac{1}{49}$ $\frac{5}{14}$ 2 $\frac{5}{18}$ $\frac{9}{16}$ $\frac{1}{4}$ $\frac{4}{5}$ $\frac{2}{5}$ $\frac{21}{2}$ $\frac{11}{42}$ $\frac{9}{14}$ $\frac{1}{15}$ $\frac{3}{4}$ 2

$3 - \frac{5}{4} = \frac{12}{4} - \frac{5}{4} = \frac{7}{4} = 1\frac{3}{4}$
$\frac{21}{20} ÷ \frac{3}{5} = \frac{21}{20} × \frac{5}{3} = \frac{7}{4}$
$\frac{3}{5} - \frac{7}{20} = \frac{12}{20} - \frac{7}{20} = \frac{5}{20} = \frac{1}{4}$
$\frac{4}{9} × \frac{1}{8} = \frac{4 × 1}{9 × 8} = \frac{1}{18}$
$\frac{33}{35} × \frac{5}{11} = \frac{33 × 5}{35 × 11} = \frac{3}{7}$
$\frac{1}{4} ÷ \frac{5}{24} = \frac{1}{4} × \frac{24}{5} = \frac{6}{5}$
$\frac{1}{2} + \frac{3}{5} = \frac{5}{10} + \frac{6}{10} = \frac{11}{10}$
$\frac{3}{7} ÷ 21 = \frac{3}{7} × \frac{1}{21} = \frac{1}{49}$
$\frac{3}{4} × \frac{10}{21} = \frac{3 × 10}{4 × 21} = \frac{30}{84} = \frac{5}{14}$
$10 × \frac{1}{5} = 2$
$\frac{2}{3} × \frac{5}{12} = \frac{2 × 5}{3 × 12} = \frac{10}{36} = \frac{5}{18}$
$\frac{7}{16} + \frac{1}{8} = \frac{7}{16} + \frac{2}{16} = \frac{9}{16}$
$\frac{3}{4} - \frac{1}{2} = \frac{3}{4} - \frac{2}{4} = \frac{1}{4}$
$\frac{4}{15} ÷ \frac{1}{3} = \frac{4}{15} × 3 = \frac{4}{5}$
$\frac{1}{3} × \frac{6}{5} = \frac{1 × 6}{3 × 5} = \frac{2}{5}$
$4 ÷ \frac{8}{21} = 4 × \frac{21}{8} = \frac{21}{2}$
$\frac{3}{7} - \frac{1}{6} = \frac{18}{42} - \frac{7}{42} = \frac{11}{42}$
$\frac{3}{10} ÷ \frac{7}{15} = \frac{3}{10} × \frac{15}{7} = \frac{45}{70} = \frac{9}{14}$
$\frac{4}{9} × \frac{3}{20} = \frac{4 × 3}{9 × 20} = \frac{12}{180} = \frac{1}{15}$
$\frac{9}{16} ÷ \frac{3}{4} = \frac{9}{16} × \frac{4}{3} = \frac{36}{48} = \frac{3}{4}$
$\frac{3}{5} × \frac{10}{3} = 2$

二、计算下面各题,怎样简便就怎样计算。
$4-(\frac {2}{5}+\frac {1}{3})÷\frac {4}{5}$
$\frac {5}{7}×\frac {3}{8}+\frac {2}{7}×\frac {3}{8}$
$88×(\frac {3}{8}+\frac {1}{44})$
$6-(\frac {6}{7}÷\frac {3}{14}+\frac {3}{16})$
$\frac {15}{7}+\frac {1}{3}÷\frac {1}{5}+\frac {1}{3}$
$\frac {8}{49}÷[\frac {10}{9}-(\frac {3}{7}+\frac {1}{9})]$

二、$\frac{37}{12}$ $\frac{3}{8}$ 35 $1\frac{13}{16}$ $\frac{29}{7}$ $\frac{2}{7}$

1. $4 - (\frac{2}{5} + \frac{1}{3}) ÷ \frac{4}{5}$
解：原式$=4 - (\frac{6}{15} + \frac{5}{15}) × \frac{5}{4}$
$=4 - \frac{11}{15} × \frac{5}{4}$
$=4 - \frac{11}{12}$
$=\frac{48}{12} - \frac{11}{12}$
$=\frac{37}{12}$
2. $\frac{5}{7} × \frac{3}{8} + \frac{2}{7} × \frac{3}{8}$
解：原式$=(\frac{5}{7} + \frac{2}{7}) × \frac{3}{8}$
$=1 × \frac{3}{8}$
$=\frac{3}{8}$
3. $88 × (\frac{3}{8} + \frac{1}{44})$
解：原式$=88 × \frac{3}{8} + 88 × \frac{1}{44}$
$=33 + 2$
$=35$
4. $6 - (\frac{6}{7} ÷ \frac{3}{14} + \frac{3}{16})$
解：原式$=6 - (\frac{6}{7} × \frac{14}{3} + \frac{3}{16})$
$=6 - (4 + \frac{3}{16})$
$=6 - 4 - \frac{3}{16}$
$=2 - \frac{3}{16}$
$=\frac{32}{16} - \frac{3}{16}$
$=\frac{29}{16} = 1\frac{13}{16}$
5. $\frac{15}{7} + \frac{1}{3} ÷ \frac{1}{5} + \frac{1}{3}$
解：原式$=\frac{15}{7} + \frac{1}{3} × 5 + \frac{1}{3}$
$=\frac{15}{7} + \frac{5}{3} + \frac{1}{3}$
$=\frac{15}{7} + (\frac{5}{3} + \frac{1}{3})$
$=\frac{15}{7} + 2$
$=\frac{15}{7} + \frac{14}{7}$
$=\frac{29}{7}$
6. $\frac{8}{49} ÷ [\frac{10}{9} - (\frac{3}{7} + \frac{1}{9})]$
解：原式$=\frac{8}{49} ÷ [\frac{10}{9} - \frac{1}{9} - \frac{3}{7}]$
$=\frac{8}{49} ÷ [1 - \frac{3}{7}]$
$=\frac{8}{49} ÷ \frac{4}{7}$
$=\frac{8}{49} × \frac{7}{4}$
$=\frac{2}{7}$