【答案】：
$? =\frac 35×4×\frac 52?$
$? =\frac 35×\frac 52×4?$
$? =\frac 32×4?$
? =6?
$? =\frac 7{12}×\frac {15}{14}×\frac 43?$
$? =\frac 58×\frac 43?$
$? =\frac 56?$
$? =\frac 7{15}×\frac 5{14}×\frac 32?$
$? =\frac 16×\frac 32?$
$? =\frac 14?$
$? =\frac 9{34}×\frac {17}2×\frac 89?$
$? =\frac 94×\frac 89?$
? =2?
【解析】：
$\frac{3}{5}×4÷\frac{2}{5}$
$=\frac{3}{5}×4×\frac{5}{2}$
$=\frac{3}{5}×\frac{5}{2}×4$
$=\frac{3}{2}×4$
$=6$
$\frac{7}{12}÷\frac{14}{15}÷\frac{3}{4}$
$=\frac{7}{12}×\frac{15}{14}×\frac{4}{3}$
$=\frac{5}{8}×\frac{4}{3}$
$=\frac{5}{6}$
$\frac{7}{15}÷\frac{14}{5}×\frac{3}{2}$
$=\frac{7}{15}×\frac{5}{14}×\frac{3}{2}$
$=\frac{1}{6}×\frac{3}{2}$
$=\frac{1}{4}$
$\frac{9}{34}÷\frac{2}{17}÷\frac{9}{8}$
$=\frac{9}{34}×\frac{17}{2}×\frac{8}{9}$
$=\frac{9}{4}×\frac{8}{9}$
$=2$
