I think they mean something like:
V = A*base_area*height, with A some constant. Indeed, A=1/3, but it does not really matter here.
You may take ratios:
V_2/V_1 = base_area_2*height_2/(base_area_1*height_1)
So, V_2/24 = 15/24*7/3 --> V_2 = 24 * 15 *7 / 24 / 3 = 35 cubic inches
This way of taking ratios of functions with some dependencies is good to know because it is asked several times.