问两道PREP的数学题

想个半天还是不会做，请高人指教

1.what is the greatest prime factor of ( 4的17次方 减去 2的28次方）？ -因为幂指数没法打出来，直接用中文了

A 2       B 3      C 5      D 7    E 11

答案是D

2. A store purchesed 20 coats that each cost equal amount and then sold each of the coats at an equal price,

what was the store's gross profit on the 20 coats?

(1) If the selling price per coat had been twice as much, the store's gross profit on the 20 coats would have been $2400

(2) If the selling price per coat had been $2 more, the store's gross profit on the 20 coats would have been $440

答案B

twice as much是指售价翻一倍么？

第二题：

if x=cost, y=price题目求：(y-x)*20

依据b,(y+2)-x=440/20，这样就可以解得(y-x)了。

twice as much是指2倍的price,依据a,2y*20-20x=2400，这样得不出(y-x)。

谢谢2楼。。。。明白了。。。

第一道自己想明白了。

4*17-2*28= 2*34-2*28= 2*28（2*6-1）=2*28×63    *代表幂

2的28次方不能被2以上的质数整除，63最大可以被7整除，多以答案是7