请教一个简单的数学题

孩子有个数学题来问我，原题是：

Elaine makes a $4 purchase and gives the cashier a $20 bill.  If she has an equal probability of receiving any of the possible combinations of bills(out of $1, $5, or $10 bills)for change, what is the probability that she will receive a $5 bill among her change?

答案选择：

A. 1/8  B. 1/4  C. 3/8  D. 1/2  E. 5/8

我和他的答案都是 １／３，可不在答案选择中。请高手指点。多谢先！

所有跟帖： 

• 回复：B? -Africangrey- ♀ (0 bytes) () 08/14/2012 postreply 10:00:04

• 回复：请教一个简单的数学题 -yanny88- ♀ (11 bytes) () 08/14/2012 postreply 10:22:24

• 不知道组合有没有限制？ -ca981- ♂ (164 bytes) () 08/14/2012 postreply 10:27:43

• This is exactly what my kid got too -coolmom- ♀ (215 bytes) () 08/14/2012 postreply 10:34:25

• 2/3. 没有这个选项阿 -bangbang9814- ♀ (0 bytes) () 08/14/2012 postreply 10:59:00

• 可能是理解问题 -yddad- ♂ (203 bytes) () 08/14/2012 postreply 13:01:12