[問題]PP的問題

[snood1014]

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

1) y=10
2) z=16

答案是D, 為什麼答案不是C呢?
請各位幫我解答!!謝謝

[FrankChang]

z=x+y,1)或2)都只會留下一個未知數x,所以都充分

[Ivy-Canada]

z=x+y,1)或2)都只會留下一個未知數x,所以都充分

再請問一下, 用 1)看來z=x+10 還是有兩個未知數 x和z. 用2)來看 16=x+y 也是有兩個未知數啊. 怎麼算出只有一個未知數x呢? 可以再解釋一下嗎? 謝謝.

[chris8888]

是這樣的
根據題目的意思化成公式 :
(0.1x + 0.02y) / (x + y) = 0.05

(1). 帶入y=10, 可以算出x, so sufficient

(2). 帶入x+y=16 and y = 16 - x
(0.1x + 0.02(16-x)) / 16 = 0.05 一樣可以求得x