2道数学机经求帮助

zjf

1. 60 cars, either full-size or compact or sports. 31 is white, 10 is red, 19 is blue, how many cars are full-size?
(1). Not all red and blue cars are full-size.
(2). The number of full-size is greater than the sum of compact and sport cars.
[确定]Ans.: C
我感觉都推不出呀，选E？

2. a group of children, each has either 3 pens and 1pencils or 1 pen and 2 pencils. The arithmetic mean of pencils per child is 1.6, what's the arithmetic mean of pens.
[确定]Ans.: 1.8
这是怎么解的呢？

Louisa_xy

1.我想也是e
2.设有3 pens and 1pencil的小孩有 m 个
    有1 pen and 2 pencils的小孩有 n个
铅笔的数量=m+2n    平均数= （m+2n）/（m+n）=1.6  可以得出m/n=3/2
钢笔的数量=3m+n    平均数= （3m+n）/（m+n） 把 m/n=3/2代入  得钢笔的平均数1.8