Help： GMAT PREP的两题数学

1. A certain stock exchange designates each stock with a one-,two-, or three-letter code, where each letter is selected from the 26 letters of the alphabet. If the letters may be repeated and if the same letters used in a different order constitute a different code, how many different stocks is it possible to uniquely designate with these codes.

The answer is 18,278

2. A used-car dealer sold one car at a profit of 25 percent of the dealer’s purchase price for that car and sold another car at a loss of 20 percent of the dealer’s purchase price for that car. If the dealer sold each car for $20,000, what was the dealer’s total profit, in dollars, for the two transactions combined?

$1,000 loss

And How?

1.题意相当于问 从26个不同的字母中 分别 取 1，2，3个的 可重复的排列一共有多少种情况。

取1个 26种

取两个 相当于两次都有26个选择，26×26=676种

取3个  相当于3次都有26个选择，26×26×26=17576种

一共26+676+17576=18278

2.设第一辆买价a，第二辆买价b

（1+25%）a=20000

（1-20%）b=20000

得a=16000

b=25000

故第一辆profit=4000

第二辆loss=5000

合起来赔1000

谢谢你的回复。原来一开始是我没看理解题目的意思