三、算一算。
1. 直接写出得数。
$\frac{1}{8} + \frac{5}{8} =$
$\frac{9}{11} - \frac{6}{11} =$
$\frac{1}{4} - \frac{1}{7} =$
$1 - \frac{1}{6} - \frac{5}{6} =$
$0.4^3 =$
$2 - \frac{5}{7} =$
$0.5 + \frac{1}{5} =$
$\frac{5}{9} - \frac{3}{8} + \frac{4}{9} =$

$\frac{1}{8} + \frac{5}{8} = \frac{3}{4}$
$\frac{9}{11} - \frac{6}{11} = \frac{3}{11}$
$\frac{1}{4} - \frac{1}{7} = \frac{3}{28}$
$1 - \frac{1}{6} - \frac{5}{6} = 0$
$0.4^3 = 0.064$
$2 - \frac{5}{7} = 1\frac{2}{7}$
$0.5 + \frac{1}{5} = 0.7$
$\frac{5}{9} - \frac{3}{8} + \frac{4}{9} = \frac{5}{8}$

2. 计算下面各题。
$\frac{4}{9} + \frac{1}{2} - \frac{5}{6}$
$\frac{1}{4} + \frac{9}{7} + \frac{1}{8} + \frac{5}{7}$
$\frac{7}{5} - (\frac{3}{7} + \frac{2}{5})$

$\frac{4}{9}+\frac{1}{2}-\frac{5}{6}$
$=\frac{8}{18}+\frac{9}{18}-\frac{15}{18}$
$=\frac{2}{18}$
$=\frac{1}{9}$
$\frac{1}{4}+\frac{9}{7}+\frac{1}{8}+\frac{5}{7}$
$=(\frac{1}{4}+\frac{1}{8})+(\frac{9}{7}+\frac{5}{7})$
$=\frac{3}{8}+2$
$=2\frac{3}{8}$
$\frac{7}{5}-(\frac{3}{7}+\frac{2}{5})$
$=\frac{7}{5}-\frac{2}{5}-\frac{3}{7}$
$=1-\frac{3}{7}$
$=\frac{4}{7}$

3. 解方程。
$x + \frac{7}{10} = \frac{4}{5}$
$2x - \frac{9}{11} = 3\frac{2}{11}$
$5 × (2.5 + x) = 21.5$

解：$x + \frac{7}{10} = \frac{4}{5}$
$x = \frac{4}{5} - \frac{7}{10}$
$x = \frac{8}{10} - \frac{7}{10}$
$x = \frac{1}{10}$
解：$2x - \frac{9}{11} = 3\frac{2}{11}$
$2x = 3\frac{2}{11} + \frac{9}{11}$
$2x = 4$
$x = 4÷2$
$x = 2$
解：$5×(2.5 + x) = 21.5$
$2.5 + x = 21.5÷5$
$2.5 + x = 4.3$
$x = 4.3 - 2.5$
$x = 1.8$

四、解决问题。
1. 明明在学校图书馆借了一本《百科全书》，共216页。他第一天看了全书的$\frac{1}{4}$，第二天看了全书的$\frac{1}{3}$，还剩这本书的几分之几没有看？

1. $1 - (\frac{1}{4} + \frac{1}{3}) = \frac{5}{12}$