(2)求未知数$x$。（6 分）
$x-\frac{3}{4}x = 80$
$x：8 = \frac{1}{2000}$
$\frac{2}{3}：x = \frac{9}{10}：\frac{3}{5}$
$x-\frac{3}{4}x = 80$
$x：8 = \frac{1}{2000}$
$\frac{2}{3}：x = \frac{9}{10}：\frac{3}{5}$
答案：4 (2)320 $\dfrac{1}{250}$ $\dfrac{4}{9}$
(3)计算下面各题。（能简算的题要简算）（12 分）
$\frac{4}{5}×0.25+\frac{1}{4}×3.2$
$\frac{13}{5}×8×\frac{8}{13}×1.25$
$4-\frac{4}{5}÷4-\frac{4}{5}$
$\frac{3}{8}÷[(\frac{1}{5}+\frac{1}{4})×\frac{3}{8}]$
$\frac{4}{5}×0.25+\frac{1}{4}×3.2$
$\frac{13}{5}×8×\frac{8}{13}×1.25$
$4-\frac{4}{5}÷4-\frac{4}{5}$
$\frac{3}{8}÷[(\frac{1}{5}+\frac{1}{4})×\frac{3}{8}]$
答案：4 (3)1 16 3 $\dfrac{20}{9}$