【答案】:
$?=\frac {1}{9}×\frac {5}{3}?$
$?=\frac {5}{27}?$
$?\frac {7}{9}×\frac {9}{21}×13?$
$?=\frac {1}{3}×13?$
$?=\frac {13}{3}?$
$?=\frac {7}{12}×\frac {8}{21}?$
$?=\frac {2}{9}?$
$?=\frac {7}{10}×\frac {15}{28}?$
$?=\frac {3}{8}?$
【解析】:
$\frac{5}{6}×\frac{2}{15}×\frac{5}{3}=\frac{5×2×5}{6×15×3}=\frac{50}{270}=\frac{5}{27}$
$\frac{1}{9}×7×13×\frac{9}{21}=\left(\frac{1}{9}×\frac{9}{21}\right)×7×13=\frac{1}{21}×7×13=\frac{1}{3}×13=\frac{13}{3}$
$\frac{1}{4}×\frac{7}{3}×\frac{8}{21}=\frac{1×7×8}{4×3×21}=\frac{56}{252}=\frac{2}{9}$
$\frac{4}{5}×\frac{7}{8}×\frac{15}{28}=\frac{4×7×15}{5×8×28}=\frac{420}{1120}=\frac{3}{8}$