[求助]请教一道prep数学

a contractor combined x tons of a gravel mixture that contains 10% gravel G, by weight, with y tons of a gravel mixture that contains 2% gravel G, to produce z tons of a gravel  that was 5% gravel G, by weight. What is the value of x?

(1) y=10

(2) z=16

我选了e ，答案是d。

选d应该如何解释啊？  谢谢