简单的数学题，但是我没读懂

thumbbunny

Three grades of milk are 1 percent, 2 percent, and 3 percent fat by volume.  If x gallons of the 1 percent grade, y gallons of the 2 percent grade, 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 and z ?

(A) y + 3z
(B) (y + z) /4
(C) 2y + 3z
(D) 3y + z
(E) 3y + 4.5z

shyl

答案是A

意思是 三种牛奶 脂肪含量分别为 1%，2%，3%。 如果X加仑的1%牛奶，和Y加仑2%的牛奶，Z加仑3%的牛奶混合后 为1.5%的脂肪含量 问如果用含Y,Z的式子表示X 应为什么？


0.01*X +0.02*Y+0.03*z= 0.015*X+0.015*Y+0.015*Z

化简得就完了