So I was reading Polya's book and in it, there was a problem involving finding the rate of change of depth of water in a cone.
At some point, we come to the conclusion that V = $\pi a^2 y^3/(3b^2)$ where $V$ = volume of water, $a$= radius of base of cone, $y$ = depth of water and $b$ = height of cone.
This is where I get lost...They then come to the conclusion that since $V$ increases along with $y$, then upon differentiation, the result should be $dV/dt = (\pi a^2 y^2 / b^2 )dy/dt$.
How come they're multiplying with $dy/dt$? whats the rule ? I know this must be extremely basic but it's been a while since I touched maths and wanted to get back into it.