I need help with this question on my assignment:

a water trough is $\displaystyle 10m$ long and a cross-section has the shape of an isosceles triangle that is $\displaystyle 1m$ across at the top and is $\displaystyle 50cm$ high. The trough is being filled with water at a rate of $\displaystyle 0.4m^3/min$. How fast will the water level rise when the water is $\displaystyle 40cm$ deep?

Please and thank you.