算一算
$9 ÷ \dfrac{7}{18} =$
$18 ÷ \dfrac{1}{6} =$
$36 ÷ \dfrac{1}{2} =$
$24 ÷ \dfrac{2}{3} =$
$1 ÷ \dfrac{5}{7} =$
$6 ÷ \dfrac{2}{7} =$
$9 ÷ \dfrac{3}{4} =$
$100 ÷ \dfrac{2}{3} =$
$88 ÷ \dfrac{22}{3} =$
$12 ÷ \dfrac{2}{3} =$

9 ÷ $\dfrac{7}{18}$ = 9 × $\dfrac{18}{7}$ = $\dfrac{162}{7}$
18 ÷ $\dfrac{1}{6}$ = 18 × 6 = 108
36 ÷ $\dfrac{1}{2}$ = 36 × 2 = 72
24 ÷ $\dfrac{2}{3}$ = 24 × $\dfrac{3}{2}$ = 36
1 ÷ $\dfrac{5}{7}$ = 1 × $\dfrac{7}{5}$ = $\dfrac{7}{5}$
6 ÷ $\dfrac{2}{7}$ = 6 × $\dfrac{7}{2}$ = 21
9 ÷ $\dfrac{3}{4}$ = 9 × $\dfrac{4}{3}$ = 12
100 ÷ $\dfrac{2}{3}$ = 100 × $\dfrac{3}{2}$ = 150
88 ÷ $\dfrac{22}{3}$ = 88 × $\dfrac{3}{22}$ = 12
12 ÷ $\dfrac{2}{3}$ = 12 × $\dfrac{3}{2}$ = 18

一、算一算。
$7x + 8x =$
$5y + 12y =$
$b + 10b =$
$8.9x - 4.2x =$
$a - 0.5a =$
$9x - 2×3x =$
$2m + 3.5m =$
$n + 2n + 3n + 4n =$

$7x + 8x = 15x$
$5y + 12y = 17y$
$b + 10b = 11b$
$8.9x - 4.2x = 4.7x$
$a - 0.5a = 0.5a$
$9x - 2×3x = 3x$
$2m + 3.5m = 5.5m$
$n + 2n + 3n + 4n = 10n$

二、解方程。
$5.4y + y = 12.8$
$6 ÷ 0.3 - 4x = 12.4$
$8x - 3x = 27.7 - 4.8$
$13 - 2x = x + 1$

$5.4y + y = 12.8$，解得$y = 2$
$6 ÷ 0.3 - 4x = 12.4$，解得$x = 1.9$
$8x - 3x = 27.7 - 4.8$，解得$x = 4.58$
$13 - 2x = x + 1$，解得$x = 4$

三、分数巧计算。
有一串分数：
$\frac{1}{2};\frac{1}{3},\frac{2}{3};\frac{1}{4},\frac{2}{4},\frac{3}{4};\frac{1}{5},\frac{2}{5},\frac{3}{5},\frac{4}{5};\frac{1}{6},\frac{2}{6},\frac{3}{6},\frac{4}{6},\frac{5}{6};···$。
请问$\frac{5}{21}$是第几个分数？第200个分数是多少？

$\frac{5}{21}$是第195个分数，第200个分数是$\frac{10}{21}$。