how much whipping cream and 2% milk should be mixed to obtain 35 gallons of milk with 4% butterfat.

Question

if the milk has 2% butterfat and the whipping cream has 9% butterfat

Answer 1

M = amount of milk (gallons)
C = amount of cream (gallons)
M + C = 35
0.02M + C = 0.04 (M + C)

Solve those two equations for M and C.