Let Length of arc, radii and angle subtended at center of circles being $l_1,\,r_1,\,\theta_1$ and $l_2,\,r_2,\,\theta_2$ respectively.

Given $\theta_1\,=\,60^0$ and $\theta_2\,=\,75^0$

Since $l_1\,=\,l_2$ ⇒ $r_1\theta_1$ = $r_2\theta_2$

Hence $r_1\times 60^0$ = $r_2\times 75^0$

Therefore $\cfrac{r_1}{r_2}$ = $\cfrac{75}{60}$

Therefore $\cfrac{r_1}{r_2}$ = $\cfrac{5}{4}$

Hence radii are in ratio of 5:4.