Arcs of the same length in two different circles subtend 40° and 50° at their centres…

With $\theta$ in radians, an arc is $s = r\theta$ and a sector has the area $A = \frac{1}{2}r^2\theta = \frac{1}{2}rs$. A rolling wheel moves $2\pi r$ in each turn. For $n$ revolutions each minute, $\omega = \frac{2\pi n}{60}$ rad/s, and a point on the rim moves at $v = r\omega$.