一、口算。
$1.78 + 0.3 =$ $0.7 + 2.5 =$ $0.8×1.4 =$
$7.6÷1.9 =$ $3.6 - 1.98 =$ $7.5÷0.25 =$
$4.23 - 1.92 =$ $2.8÷0.02 =$ $1.25×3 =$
$0.35×0.4 =$ $4.57 + 6.43 =$ $17.2 - 8.5 =$
$\frac{1}{5} + \frac{11}{24} =$ $6 - \frac{8}{11} =$ $\frac{1}{15} + \frac{4}{15} =$
$\frac{3}{4} - \frac{1}{9} =$ $\frac{2}{7} + \frac{3}{8} =$ $6 + \frac{1}{8} =$
$\frac{3}{8} + \frac{2}{5} =$ $\frac{5}{7} + \frac{2}{9} =$ $\frac{4}{5} - \frac{2}{3} =$
$\frac{7}{8} + \frac{1}{2} =$ $\frac{9}{10} - \frac{1}{3} =$ $\frac{3}{5} + \frac{7}{11} =$

$1.78 + 0.3 = 2.08$；$0.7 + 2.5 = 3.2$；$0.8×1.4 = 1.12$；
$7.6÷1.9 = 4$；$3.6 - 1.98 = 1.62$；$7.5÷0.25 = 30$；
$4.23 - 1.92 = 2.31$；$2.8÷0.02 = 140$；$1.25×3 = 3.75$；
$0.35×0.4 = 0.14$；$4.57 + 6.43 = 11$；$17.2 - 8.5 = 8.7$；
$\frac{1}{5} + \frac{11}{24} = \frac{24}{120} + \frac{55}{120} = \frac{79}{120}$；$6 - \frac{8}{11} = \frac{66}{11} - \frac{8}{11} = \frac{58}{11}$；$\frac{1}{15} + \frac{4}{15} = \frac{5}{15} = \frac{1}{3}$；
$\frac{3}{4} - \frac{1}{9} = \frac{27}{36} - \frac{4}{36} = \frac{23}{36}$；$\frac{2}{7} + \frac{3}{8} = \frac{16}{56} + \frac{21}{56} = \frac{37}{56}$；$6 + \frac{1}{8} = 6\frac{1}{8}$；
$\frac{3}{8} + \frac{2}{5} = \frac{15}{40} + \frac{16}{40} = \frac{31}{40}$；$\frac{5}{7} + \frac{2}{9} = \frac{45}{63} + \frac{14}{63} = \frac{59}{63}$；$\frac{4}{5} - \frac{2}{3} = \frac{12}{15} - \frac{10}{15} = \frac{2}{15}$；
$\frac{7}{8} + \frac{1}{2} = \frac{7}{8} + \frac{4}{8} = \frac{11}{8}$；$\frac{9}{10} - \frac{1}{3} = \frac{27}{30} - \frac{10}{30} = \frac{17}{30}$；$\frac{3}{5} + \frac{7}{11} = \frac{33}{55} + \frac{35}{55} = \frac{68}{55}$。

二、计算下面各题，怎样简便就怎样计算。
$\frac{2}{3}×\frac{13}{8}×\frac{4}{39}$         $\frac{11}{8} - (\frac{6}{7} + \frac{3}{8})$         $\frac{13}{7} + \frac{2}{15} + \frac{1}{7} + \frac{13}{15}$
$\frac{40}{13}×\frac{7}{80}×39$         $\frac{3}{10} + \frac{5}{8} - \frac{4}{5}$         $5 - \frac{1}{8} - \frac{5}{9} - \frac{7}{8}$

1. 计算$\frac{2}{3}×\frac{13}{8}×\frac{4}{39}$：
解：
$\frac{2}{3}×\frac{13}{8}×\frac{4}{39}=\frac{2×13×4}{3×8×39}=\frac{2×13×4}{3×4×2×39}=\frac{1}{9}$
2. 计算$\frac{11}{8}-(\frac{6}{7}+\frac{3}{8})$：
解：
$\frac{11}{8}-(\frac{6}{7}+\frac{3}{8})=\frac{11}{8}-\frac{3}{8}-\frac{6}{7}=1 - \frac{6}{7}=\frac{1}{7}$
3. 计算$\frac{13}{7}+\frac{2}{15}+\frac{1}{7}+\frac{13}{15}$：
解：
$(\frac{13}{7}+\frac{1}{7})+(\frac{2}{15}+\frac{13}{15}) = 2 + 1=3$
4. 计算$\frac{40}{13}×\frac{7}{80}×39$：
解：
$\frac{40}{13}×\frac{7}{80}×39=\frac{40×7×39}{13×80}=\frac{40×7×3}{80}=\frac{21}{2}$
5. 计算$\frac{3}{10}+\frac{5}{8}-\frac{4}{5}$：
解：
先通分，$10$、$8$、$5$的最小公倍数是$40$。
$\frac{3×4}{10×4}+\frac{5×5}{8×5}-\frac{4×8}{5×8}=\frac{12}{40}+\frac{25}{40}-\frac{32}{40}=\frac{12 + 25-32}{40}=\frac{5}{40}=\frac{1}{8}$
6. 计算$5-\frac{1}{8}-\frac{5}{9}-\frac{7}{8}$：
解：
$(5-\frac{5}{9})-(\frac{1}{8}+\frac{7}{8})=\frac{45 - 5}{9}-1=\frac{40}{9}-1=\frac{40 - 9}{9}=\frac{31}{9}$
综上，答案依次为$\frac{1}{9}$；$\frac{1}{7}$；$3$；$\frac{21}{2}$；$\frac{1}{8}$；$\frac{31}{9}$。