四、算一算。
1. 把$\frac{2}{5}$变换成分母是20且大小不变的分数。

1. $\frac{2}{5}=\frac{2×4}{5×4}=\frac{8}{20}$

2. 把$\frac{3}{18}$变换成分子是1且大小不变的分数。

2. $\frac{3}{18}=\frac{3÷3}{18÷3}=\frac{1}{6}$

五、解决问题。

他们俩谁做题更快些？

$11÷5=\frac{11}{5}$ $7÷3=\frac{7}{3}$
$\frac{11}{5}＜\frac{7}{3}$ 小松做题更快些。

算一算
$\frac{3}{8} + \frac{4}{9} =$
$\frac{2}{6} + \frac{4}{7} =$
$\frac{2}{7} + \frac{5}{8} =$
$\frac{7}{15} + \frac{4}{5} =$
$\frac{5}{12} + \frac{7}{18} =$
$\frac{1}{2} - \frac{1}{3} =$
$\frac{1}{5} - \frac{1}{6} =$
$\frac{1}{8} - \frac{1}{9} =$
$\frac{3}{4} - \frac{2}{3} =$
$\frac{5}{6} - \frac{3}{5} =$

$\frac{3}{8} + \frac{4}{9} = \frac{27}{72} + \frac{32}{72} = \frac{59}{72}$
$\frac{2}{6} + \frac{4}{7} = \frac{1}{3} + \frac{4}{7} = \frac{7}{21} + \frac{12}{21} = \frac{19}{21}$
$\frac{2}{7} + \frac{5}{8} = \frac{16}{56} + \frac{35}{56} = \frac{51}{56}$
$\frac{7}{15} + \frac{4}{5} = \frac{7}{15} + \frac{12}{15} = \frac{19}{15}$
$\frac{5}{12} + \frac{7}{18} = \frac{15}{36} + \frac{14}{36} = \frac{29}{36}$
$\frac{1}{2} - \frac{1}{3} = \frac{3}{6} - \frac{2}{6} = \frac{1}{6}$
$\frac{1}{5} - \frac{1}{6} = \frac{6}{30} - \frac{5}{30} = \frac{1}{30}$
$\frac{1}{8} - \frac{1}{9} = \frac{9}{72} - \frac{8}{72} = \frac{1}{72}$
$\frac{3}{4} - \frac{2}{3} = \frac{9}{12} - \frac{8}{12} = \frac{1}{12}$
$\frac{5}{6} - \frac{3}{5} = \frac{25}{30} - \frac{18}{30} = \frac{7}{30}$