A interesting story.
A nine-year-old boy had a ache in his teeth and it turned out to be so bad that it must be pulled out.(九岁的男孩牙痛,而且结果那颗牙已经坏到非拔掉不可的程度了)So the mother brought his son to the dentist's to pull out his bad teeth.Before the operation,the dentist made a deal with the mother on the price to be four dollars.But just after the operation the dentist charged the mother for twenty dollars,four times than they had talked before.The mother asked why he said so.The dentist said"Well,your son's cry was so fearful that other four patients were scared away.(你儿子的叫声太吓人了,结果其他四个病人都给吓跑了)
基本上按照你给出的原文,添加指出已给出解释.