If in two circles, arcs of the same length subtend angles 60° and 75° at the centre,

Let the radii of the two circles be $r_{1}$ and $r_{2}$. Let an arc of length / subtend an angle of $60^{\circ}$ at the centre of the circle of radius $r_{1}$, while let an arc of length / subtend an angle of $75^{\circ}$ at the centre of the circle of radius $r_{2}$.

Now, $60^{\circ}=\frac{\pi}{3}$ radian and $75^{\circ}=\frac{5 \pi}{12}$ radian

We know that in a circle of radius $r$ unit, if an arc of length / unit subtends an angle $\theta$ radian at the centre, then $\theta=\frac{l}{r}$ or $l=r \theta$.

$\therefore l=\frac{r_{1} \pi}{3}$ and $l=\frac{r_{2} 5 \pi}{12}$

$\Rightarrow \frac{r_{1} \pi}{3}=\frac{r_{2} 5 \pi}{12}$

$\Rightarrow r_{1}=\frac{r_{2} 5}{4}$

$\Rightarrow \frac{r_{1}}{r_{2}}=\frac{5}{4}$

Thus, the ratio of the radii is $5: 4$.