Mezarashi, because it say an inverted isosceles triangle, I interpret this as meaning that the vertex of the triangle is at the bottom so prism widens toward the top, not narrow.
mathmann, (1/2)bhl is the volume of the entire prism. Draw a picture of the triangle and draw a horizontal line in it representing the water level. It is the volume of the water that you want. Since h is increasing, "b", the length of the base is also increasing. You need to replace b by a function of h.
To do that, look at your picture. You should see that you have two similar triangles- the triangle formed by the end of the prism and the triangle formed by the water. They have exactly the same angles and so are similar. That means that corresponding lenghts are in the same proportions. The top of the prism, divided by the height, 60/40, is equal to the top of the water, b, divided by the height of the water, h: b/h= 60/40. That easily gives you "b" as a function of "h". Replace b in your calculation by that function.
By the way, what course is this? You seem to be consistently posting, in "precalculus", problems that I would consider "calculus".