四、计算。
1. 直接写得数。
$\frac{1}{10} ÷ \frac{1}{4} =$
$14 ÷ \frac{7}{8} =$
$6 × 50\% =$
$\frac{1}{3} × \frac{3}{5} ÷ \frac{5}{18} =$
$14 × \frac{3}{7} =$
$\frac{1}{6} × \frac{2}{7} × 6 =$
$\frac{11}{14} ÷ \frac{11}{28} =$
$1 ÷ \frac{1}{3} × \frac{2}{3} =$
$3.6 × \frac{4}{9} =$
$\frac{7}{15} ÷ 21 =$
$\frac{1}{8} - \frac{1}{4} =$
$\frac{11}{12} × 0 ÷ \frac{9}{7} =$

$\frac{1}{10} ÷ \frac{1}{4} = \frac{2}{5}$
$14 ÷ \frac{7}{8} = 16$
$6 × 50\% = 3$
$\frac{1}{3} × \frac{3}{5} ÷ \frac{5}{18} = \frac{18}{25}$
$14 × \frac{3}{7} = 6$
$\frac{1}{6} × \frac{2}{7} × 6 = \frac{2}{7}$
$\frac{11}{14} ÷ \frac{11}{28} = 2$
$1 ÷ \frac{1}{3} × \frac{2}{3} = 2$
$3.6 × \frac{4}{9} = 1.6$
$\frac{7}{15} ÷ 21 = \frac{1}{45}$
$\frac{1}{8} - \frac{1}{4} = -\frac{1}{8}$
$\frac{11}{12} × 0 ÷ \frac{9}{7} = 0$

2. 计算下面各题,怎样简便就怎样算。
$\frac{5}{7} + \frac{8}{9} - \frac{3}{7} + \frac{1}{9}$
$6 ÷ \frac{3}{7} - \frac{3}{7} ÷ 6$
$\frac{3}{4} × (2 ÷ \frac{3}{11} × \frac{8}{11})$
$18 ÷ \frac{3}{8} ÷ \frac{2}{3}$
$\frac{1}{4} ÷ \frac{9}{8} + \frac{8}{9} × \frac{3}{4}$
$(\frac{1}{2} - \frac{1}{3}) ÷ (\frac{1}{3} + \frac{1}{4})$

$\frac{5}{7} + \frac{8}{9} - \frac{3}{7} + \frac{1}{9}$
$= (\frac{5}{7} - \frac{3}{7}) + (\frac{8}{9} + \frac{1}{9})$
$= \frac{2}{7} + 1$
$= \frac{9}{7}$
$6 ÷ \frac{3}{7} - \frac{3}{7} ÷ 6$
$= 6 × \frac{7}{3} - \frac{3}{7} × \frac{1}{6}$
$= 14 - \frac{1}{14}$
$= \frac{195}{14}$
$\frac{3}{4} × (2 ÷ \frac{3}{11} × \frac{8}{11})$
$= \frac{3}{4} × 2 × \frac{11}{3} × \frac{8}{11}$
$= (\frac{3}{4} × \frac{11}{3}) × (2 × \frac{8}{11})$
$= \frac{11}{4} × \frac{16}{11}$
$= 4$
$18 ÷ \frac{3}{8} ÷ \frac{2}{3}$
$= 18 × \frac{8}{3} × \frac{3}{2}$
$= 18 × (\frac{8}{3} × \frac{3}{2})$
$= 18 × 4$
$= 72$
$\frac{1}{4} ÷ \frac{9}{8} + \frac{8}{9} × \frac{3}{4}$
$= \frac{1}{4} × \frac{8}{9} + \frac{8}{9} × \frac{3}{4}$
$= \frac{8}{9} × (\frac{1}{4} + \frac{3}{4})$
$= \frac{8}{9} × 1$
$= \frac{8}{9}$
$(\frac{1}{2} - \frac{1}{3}) ÷ (\frac{1}{3} + \frac{1}{4})$
$= \frac{1}{6} ÷ \frac{7}{12}$
$= \frac{1}{6} × \frac{12}{7}$
$= \frac{2}{7}$

3. 列式计算。
(1) $\frac{1}{3}$与$\frac{1}{4}$的差除以$\frac{5}{12}$，商是多少？
(2) 甲数的$\frac{3}{8}$是乙数的$\frac{4}{5}$，乙数是90，甲数是多少？

(1)
$(\frac{1}{3} - \frac{1}{4}) ÷ \frac{5}{12}$
$= \frac{1}{12} ÷ \frac{5}{12}$
$= \frac{1}{5}$
(2)
$90 × \frac{4}{5} ÷ \frac{3}{8}$
$= 72 ÷ \frac{3}{8}$
$= 192$
答：(1)商是$\frac{1}{5}$；(2)甲数是192。