pp的题 大大帮帮忙啊

B bongh two shares of stock,whicn he sold for $96 each, If he had a profit of 20% one the sale of one of the share but a loss of 20% one the sale of the other share, then on the sale of both shares combined B had

Key: a loss of $8

是怎么算出来的，不是两个20％的基数都一样吗？那不是应该不赚不赔？

谢谢 大大 我开始懂了