[求助]Prep破解ds1的56题

56.   4741-!-item-!-187;#058&004206

During a sale, a clothing store sold each shirt at a price of $15 and each sweater at a price of $25.  Did the store sell more sweaters than shirts during the sale?

(1)  The average (arithmetic mean) of the prices of all of the shirts and sweaters that the store sold during the sale was $21.

(2)  The total of the prices of all of the shirts and sweaters that the store sold during the sale was $420.

答案A。为什么啊

shirt no.:a,sweater no.:b

1. (15a+25b)/a+b=21 -->2b=3a--->  b>a

2. 15a+25b=420 --->  a=8,b=12或a=18,b=6 

1答案确定，选A