一、直接写出得数。
$1.3 + 4.5 =$ $\frac{1}{2} + \frac{1}{3} =$ $3.6 ÷ 4 =$ $4.5 × 2 =$ $16.7 - 5.4 =$
$\frac{2}{5} + \frac{3}{5} =$ $1 - \frac{7}{9} =$ $\frac{7}{16} - \frac{3}{8} =$ $0.9 × 0.9 =$ $81 - 3.7 =$

$1.3 + 4.5 = 5.8$
$\frac{1}{2} + \frac{1}{3} = \frac{5}{6}$
$3.6 ÷ 4 = 0.9$
$4.5 × 2 = 9$
$16.7 - 5.4 = 11.3$
$\frac{2}{5} + \frac{3}{5} = 1$
$1 - \frac{7}{9} = \frac{2}{9}$
$\frac{7}{16} - \frac{3}{8} = \frac{1}{16}$
$0.9 × 0.9 = 0.81$
$81 - 3.7 = 77.3$

二、计算下面各题。
$\frac{3}{4} - \frac{1}{8} + \frac{5}{12}$
$\frac{7}{12} - ( \frac{3}{4} - \frac{1}{2} )$
$\frac{1}{2} - ( \frac{3}{4} - \frac{3}{8} )$

$\frac{3}{4} - \frac{1}{8} + \frac{5}{12}$
$=\frac{18}{24} - \frac{3}{24} + \frac{10}{24}$
$=\frac{15}{24} + \frac{10}{24}$
$=\frac{25}{24}$
$\frac{7}{12} - ( \frac{3}{4} - \frac{1}{2} )$
$=\frac{7}{12} - ( \frac{3}{4} - \frac{2}{4} )$
$=\frac{7}{12} - \frac{1}{4}$
$=\frac{7}{12} - \frac{3}{12}$
$=\frac{4}{12}$
$=\frac{1}{3}$
$\frac{1}{2} - ( \frac{3}{4} - \frac{3}{8} )$
$=\frac{4}{8} - ( \frac{6}{8} - \frac{3}{8} )$
$=\frac{4}{8} - \frac{3}{8}$
$=\frac{1}{8}$

三、解方程。
$x - \frac{3}{8} = \frac{3}{4}$
$\frac{2}{5} + x = \frac{8}{9}$
$2.5x = 17.5$

解：
$x - \frac{3}{8} = \frac{3}{4}$
$x = \frac{3}{4} + \frac{3}{8}$
$x = \frac{6}{8} + \frac{3}{8}$
$x = \frac{9}{8}$
解：
$\frac{2}{5} + x = \frac{8}{9}$
$x = \frac{8}{9} - \frac{2}{5}$
$x = \frac{40}{45} - \frac{18}{45}$
$x = \frac{22}{45}$
解：
$2.5x = 17.5$
$x = 17.5 ÷ 2.5$
$x = 7$