79. Вычислите: а) (7/30 + 1/2 + 4/15) : (49/50 − 14/25 − 2/5); б) 39 : (3/8 + 1/6) + (3/10)² · (2/3 − 7/18).

а) \[\left(\frac7{30}+\frac12+\frac4{15}\right):\left(\frac{49}{50}-\frac{14}{25}-\frac25\right)=1:\frac1{50}=50\]

б) \[39:\left(\frac38+\frac16\right)+\left(\frac3{10}\right)^2\cdot\left(\frac23-\frac7{18}\right)\]

\[=39:\frac{13}{24}+\frac9{100}\cdot\frac5{18}=72+\frac1{40}=\frac{2881}{40}\]

Ответ: а) 50; б) \(\frac{2881}{40}\).