1. 口算。
$0.75×\frac {2}{5}=$ $\frac {3}{7}÷\frac {8}{7}=$ $\frac {1}{2}×\frac {6}{7}=$ $\frac {5}{21}×14=$
$\frac {9}{10}÷\frac {3}{5}=$ $48÷\frac {6}{19}=$ $\frac {3}{10}÷\frac {2}{5}=$ $3.2÷\frac {4}{15}=$
$9×40\% =$ $\frac {1}{4}÷50\% =$ $\frac {2}{3}÷9=$ $75\% ×16=$

0.3
$\frac{3}{8}$
$\frac{3}{7}$
$\frac{10}{3}$
$\frac{3}{2}$
152
$\frac{3}{4}$
12
3.6
0.5
$\frac{2}{27}$
12

2. 计算下面各题,能简便的请用简便方法计算。
$7.2×(\frac {5}{8}-\frac {2}{9})$ $\frac {16}{35}÷\frac {8}{15}÷\frac {2}{9}$ $\frac {3}{4}×52\% +0.75×0.48$
$\frac {1}{18}×(\frac {1}{3}+\frac {5}{12})$ $15×\frac {15}{16}$ $\frac {8}{15}×[\frac {5}{6}÷(\frac {7}{9}-\frac {1}{3})]$

$7.2×(\frac {5}{8}-\frac {2}{9})$
$=7.2×\frac {5}{8}-7.2×\frac {2}{9}$
$=4.5 - 1.6$
$=2.9$
$\frac {16}{35}÷\frac {8}{15}÷\frac {2}{9}$
$=\frac {16}{35}×\frac {15}{8}×\frac {9}{2}$
$=\frac {6}{7}×\frac {9}{2}$
$=\frac {27}{7}$
$\frac {3}{4}×52\% +0.75×0.48$
$=0.75×0.52 + 0.75×0.48$
$=0.75×(0.52 + 0.48)$
$=0.75×1$
$=0.75$
$\frac {1}{18}×(\frac {1}{3}+\frac {5}{12})$
$=\frac {1}{18}×(\frac {4}{12}+\frac {5}{12})$
$=\frac {1}{18}×\frac {9}{12}$
$=\frac {1}{18}×\frac {3}{4}$
$=\frac {1}{24}$
$15×\frac {15}{16}$
$=(16 - 1)×\frac {15}{16}$
$=16×\frac {15}{16} - 1×\frac {15}{16}$
$=15 - \frac {15}{16}$
$=14\frac {1}{16}$
$\frac {8}{15}×[\frac {5}{6}÷(\frac {7}{9}-\frac {1}{3})]$
$=\frac {8}{15}×[\frac {5}{6}÷(\frac {7}{9}-\frac {3}{9})]$
$=\frac {8}{15}×[\frac {5}{6}÷\frac {4}{9}]$
$=\frac {8}{15}×[\frac {5}{6}×\frac {9}{4}]$
$=\frac {8}{15}×\frac {15}{8}$
$=1$

3. 在植树活动中,四年级、五年级、六年级学生一共植树102棵,已知他们的植树棵数之比是$1:2:3$。
(1)四年级、五年级、六年级学生分别植树多少棵？
(2)四年级学生比六年级学生少植树百分之几？

(1)1+2+3=6
四年级：102×1/6=17(棵)
五年级：102×2/6=34(棵)
六年级：102×3/6=51(棵)
(2)(51-17)÷51≈0.667=66.7%