ROBERT 经典答疑中的疑问

gmat02

11. If x and y are positive integers such that x=8y+12,what is the greatest common divisor of x and y?
1) x=12u,where u is an integer
2) y=12z,where z is 解答：由题知: x=4(2y+3)
由(1)知: x=12u 则y=3k k is an integer. x=12(2k+1) 则x,y最大公因子不能确定.
由(2)知: y=12z 则x=12(6z+1) ,因此x,y的公因子是12。
这题我认为选Ban integer

确认是B, 对吗?

14. X的各位和是170. X=20^17-Y,问Y. 可选项是从53到6?
这个题有些类似那道减去两位数的题。x=10^17*2^17-y=131072*10^17-y
170-13=157

不明白,可否解释一下

22．On Jane’s credit card account, the average daily balance for a 30-day billing cycle is the average of the daily balances at the end of each of the 30 days.At the beginning of a certain 30-day billing cycle,Jane’s credit card account had a balance of $600.Jane made a payment of $300 on the account during the billing cycle.If no other amounts were added to or subtracted from the account during the billing cycle, what was the average daily balance on Jane’s account for the billing cycle?
1) Jane’s payment was credited on the 21st day of the billing cycle.
2) The average daily balance through the 25th day of the billing cycle was $540.
The key is D,but why?

题意不理解
25..
一道几何题，描述如下，大小两圆在大圆左侧内切，穿过两圆心B、D及切点A的直线交大圆于E、交小圆于C，问大圆内、小圆外的面积？
A. CD=1、DE=5
B. BC=2、CD=1
key
我的KEY 是B,  从A 无法得知大圆半径