[原创]GWD13-8

Q8:

At a certain food stand, the price of each apple is ¢ 40 and the price of each orange is ¢ 60.  Mary selects a total of 10 apples and oranges from the food stand, and the average (arithmetic mean) price of the 10 pieces of fruit is ¢ 56.  How many oranges must Mary put back so that the average price of the pieces of fruit that she keeps is ¢ 52?

A. 1

B. 2

C. 3

D. 4

E. 5

When the average (arithmetic mean) price of the 10 pieces of fruit is ¢ 56

apple =2, orange=8   

(0.4*2+0.6*8)/10=0.56

when the average price of the pieces of fruit that she keeps is ¢ 52

apple =4, orang=6

(0.4*4+0.6*6)/10=0.52

the key should be B (8-6=2), Why the answer gives E?

题目意思是说减少orange的数量使得平均价格降低,并不是说总数不变.