Suppose the first purchase price is x, 2*(4800/x)= 10800/(x+20), and x= 160 is obtained. The first purchase price is 160, and 30 pairs are bought: 60 pairs are bought for the second time, each pair is 60.

Total cost 15600, profit at least 15600*.2=3 120, total selling price15600+3120 =18720, at least for each pair.