Answer
$=90\bar{0}\ rpm$
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
$$
9.00\times 1500=15.0\times x\Longrightarrow x=\frac{9.00\times 1500}{15.0}=90\bar{0}\ rpm
.$$