Application of the Differentiation

"The owners of a large farm have decided to begin converting the land to soybean production. The annual soybean harvest, H, in bushels, is a product of Y, the yield per acre (bushels per acre), times A, the number of acres of soybeans planted. If Y and A as functions of t, the time in years from the beginning of the conversion are described by the equations: Y = 100 + 10 sqrt t, A = 200+100t Find the rate at which H is changing when (a) t = 1 and (b) t = 4"