还有一道GMAT数学题

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还有一道GMAT数学题
At a certain food stand, theprice of each apple is ¢ 40 and the price of each orange is ¢60. Mary selects a total of 10 applesand oranges from the food stand, and the average (arithmetic mean) price of the10 pieces of fruit is ¢ 56. Howmany oranges must Mary put back so that the average price of the pieces offruit that she keeps is ¢ 52?

A. 1
B. 2
C. 3
D. 4
E. 5
（为什么不选d,4个orange,6个apple,平均价格刚好是52呀）

Mary paid = 10(56) = ¢560
if the average price drop to ¢52,she paid =52x10 =¢520
She paid less = 560-520 =¢40
The price difference between orange and apple =60-40 = ¢20
so she must put back =40/20 = 2 pieces of orange
ans :B
再问：给的答案是E。。。。
再答： Mary paid = 10(56) = ¢560 = 8 oranges + 2 apples

let x be the number of oranges that Mary should put back

[(8-x)60 + 2(40)] /(10-x)=52
480x -60x +80 =520-52x
8x = 40
x=5
Ans: E

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