39) $$ \frac{y^{20} \cdot y^{30}}{y^{10} \cdot y \cdot y^{10}} = \frac{y^{20+30}}{y^{10+1+10}} = \frac{y^{50}}{y^{21}} = y^{50-21} = y^{29}$$
Ответ: y29