请教两道prep题

1. A certain bastket contains 10 apples , 7 of which are red and 3 of whcih are green. If 3 different apple are to be selected at random from the basket, what is the probability that 2 of the apples aelected will be red and 1 will be green?

答案是21/40

这道题应该不难，可是算不出答案, 这样想哪里有错吗, 先挑一个绿的, 概率是C(3,1)/C(10,1), 乘以红的C(7,2)/C(9,2), c

2. For Manufacturer M, the cost C of producing X Units of its product per month is given by c=kx+t, where c is in dollars and k and t are constants. Last month if Manufacturer M produced 1,000 units of its product and sold all the units for k+60 dollars each, what was Manufacturer M's gross profit on the 1,000 units?

1) Last month, Manufacturer M's revenue from the sale of the 1,000 units was 150,000

2) Manufacturer M's cost of producting 500 Units in a month is 45,000 less than its cost of producing 1,ooo units in a month

c=kx+t    last month cost=1000k+t     profit: 1000(k+60) -(1000k+t) =60000-t  ,

(1)150000=1000k+t     (2) (1000k+t)-(500k+t)=45000   得出k ,  由(1)得出t,  profit可以算出,c   可是答案是e,为什么?哪里不对?

你这样想就有了次序：绿红红。而题目是没次序的所以应该再乘3，算上红绿红，红红绿。

（1）是说的revenue，题目给的是cost的公式，式子列的不对。

应为150000=1000*(k+60)

（1）（2）都是可求出k,无法求出t.

所以没法求出利润。