[求助]princeton GMAT diagnostic里的一道题，不难，但就是没看懂。。。

A tailor trims 4 feet from opposite edges of a square piece of cloth, and 5 feet from the other 2 edges. If 120 square feet of cloth remains, what was the length of a side of the original piece of cloth?

各位NN拜托告诉我为啥答案是20呀？

我楞就是一点也不理解。。。看解释也看不懂。拜托啦！谢谢大家^^

Look at the attached graph

thanx a lot^^, very enlighted