Can someone show me how to solve this one please...

three grades of milk are 1 percent, 2 percent, and 3 percent fat by volume. If x gallons of 1 percent, y gallons of 2 percent, and z gallons of the 3 percent grade are mixed to give x+y+z gallons of a 1.5 percent grade, what is x in terms of y+z?

a) y+3z
b) (y+z)/4
c) 2y+3z
d) 3y+z
e) 3y+4.5z

(the answer is A)... thanks!