Here's another one that apparently I can't get it.

A water trough is 4 m long and its cross-section is an isosceles trapezoid which is 80 cm wide at the bottom and 120 cm wide at the top, and the height is 40 cm. The trough is not full. Give an expression for V, the volume of water in the trough in cm3, when the depth of the water is d cm.