[求助]急，涛涛的两道数学题，在线等！

各位XDJM，这道题想了n久，还是没想通，只好求助大家啦，在此先谢过了，呵呵！

Q30:

Joanna bought only $0.15 stamps and $0.29 stamps.  How many $0.15 stamps did she buy?

(1)     She bought $4.40 worth of stamps.

(2)     She bought an equal number of $0.15 stamps and $0.29 stamps.

选A，怎么求出买了多少张啊？

另外，下面这道排列组合题我是用穷举法解出来的，大家还有没有better solution,可以快点算出答案呢？

Q25:

A photographer will arrange 6 people of 6 different heights for photograph by placing them in two rows of three so that each person in the first row is standing in front of someone in the second row.  The heights of the people within each row must increase from left to right, and each person in the second row must be taller than the person standing in front of him or her.  How many such arrangements of the 6 people are possible?

答案：5

Thanks a million!!!

第一题，观察，0.15+0.29=0.44 那么各10张成立

15和29互质，10是唯一解

第二题，设123456，16不能动，第一排中间只能是2或3，2两种3两种，加上原本的一种，共五种。