4. 把30 g糖放入120 g的水中,糖占糖水的()。
①20%
②25%
③40%

30÷(30+120)=20%
答：①。

5. 一件上衣降价10%销售，“店庆”期间在此基础上再次降价5%。现价是原价的()。
①85%
②85.5%
③95%

把原价看作单位“1”
1×(1-10%)×(1-5%)
= 0.9 × 0.95
= 85.5%
答：现价是原价的85.5%，选②。

1.直接写得数。
$\frac{5}{8} × 1.6 =$ $\frac{2}{3} × \frac{3}{4} =$ $\frac{3}{5} ÷ \frac{1}{5} =$ $1 - \frac{4}{7} ÷ \frac{4}{7} =$
$3 ÷ \frac{1}{5} =$ $\frac{9}{20} ÷ 3 =$ $\frac{4}{5} × \frac{5}{9} =$ $\frac{2}{3} × \frac{3}{4} × \frac{4}{5} =$

$\frac{5}{8} × 1.6 = 1$
$\frac{2}{3} × \frac{3}{4} = \frac{1}{2}$
$\frac{3}{5} ÷ \frac{1}{5} = 3$
$1 - \frac{4}{7} ÷ \frac{4}{7} = 0$
$3 ÷ \frac{1}{5} = 15$
$\frac{9}{20} ÷ 3 = \frac{3}{20}$
$\frac{4}{5} × \frac{5}{9} = \frac{4}{9}$
$\frac{2}{3} × \frac{3}{4} × \frac{4}{5} = \frac{2}{5}$

2.怎样简便就怎样算。
$\frac{2}{5} × \frac{3}{7} + \frac{2}{5} × \frac{4}{7}$
$( \frac{1}{4} + \frac{2}{7} ) × 2.8$
$( \frac{7}{12} + \frac{3}{8} - \frac{23}{24} ) × 24$
$\frac{1}{100} × 999 + \frac{1}{100}$
$\frac{3}{8} ÷ \frac{9}{10} ÷ 6$
$\frac{3}{5} ÷ ( \frac{1}{4} + \frac{4}{5} )$

$\frac{2}{5} × \frac{3}{7} + \frac{2}{5} × \frac{4}{7}$
$=\frac{2}{5}×(\frac{3}{7}+\frac{4}{7})$
$=\frac{2}{5}×1$
$=\frac{2}{5}$
$(\frac{1}{4} + \frac{2}{7}) × 2.8$
$=\frac{1}{4}×2.8 + \frac{2}{7}×2.8$
$=0.7 + 0.8$
$=1.5$
$(\frac{7}{12} + \frac{3}{8} - \frac{23}{24}) × 24$
$=\frac{7}{12}×24 + \frac{3}{8}×24 - \frac{23}{24}×24$
$=14 + 9 - 23$
$=0$
$\frac{1}{100} × 999 + \frac{1}{100}$
$=\frac{1}{100}×(999+1)$
$=\frac{1}{100}×1000$
$=10$
$\frac{3}{8} ÷ \frac{9}{10} ÷ 6$
$=\frac{3}{8}×\frac{10}{9}×\frac{1}{6}$
$=\frac{5}{12}×\frac{1}{6}$
$=\frac{5}{72}$
$\frac{3}{5} ÷ (\frac{1}{4} + \frac{4}{5})$
$=\frac{3}{5} ÷ \frac{21}{20}$
$=\frac{3}{5}×\frac{20}{21}$
$=\frac{4}{7}$