"The price per share of a stock decreased from $90 per share to $36 per share. By what percent did the price of the stock decrease?"

First, find the amount that the stock decreased:  $90 - $36  =  $54

Now, compare that amount to the original amount by finding what part that is of the original amount (thus, you have to divide):  $54 / $90  =  0.60  =  60%