求JJ-275题思路

以下是引用qqqsunny在2008-6-17 20:30:00的发言：
5a-7b=23
a=(7b+23)/5
a-b= (7b+23)/5-b= (2b+23)/5
b>0, when b=1, a-b have the least value  5...

hope it helps...