Answer
$563\ rpm$
Work Step by Step
The relationship of two rotating pulleys connected by a belt is:
(diameter of A)(rpm of A) = (diameter of B)(rpm of B).
Now, we have
$$
25\times 900=40\times x\Longrightarrow x=\frac{25\times 900 }{40}=562.5\ rpm\approx 563\ rpm
.$$
