No. of collisions $=\frac{1}{4} n<v>=v$
Now, $\frac{v^{\prime}}{v}=\frac{n^{\prime}}{n} \frac{\left\langle v^{\prime}\right\rangle}{\langle v>}=\frac{1}{\eta} \sqrt{\frac{T}{T}}$
(When the gas is expanded $\eta$ times, $n$ decreases by a factor $\eta$ ). Also $T(\eta V)^{2 / i}=T V^{2 / i}$ or $T=\eta^{2 / i} T$ so, $\frac{v^{\prime}}{v}=\frac{1}{\eta} \eta^{-1 / i}=\eta^{\frac{-i-1}{i}}$
i.e. collisions decrease by a factor $\eta^{\frac{i+1}{i}}, i=5$ here.