Well, the local Barnes and Noble Bookstore ordered 80 books but received 24. How we should get the percentage of what's missing is to first, subtract 80 to 24. That should give you 56. Then, how you find the percentage is dividing, but you will get a decimal. So you divide 56 and 80. 56/80 should get you 0.70.
Another way to do that is to put a variable in it.
{80/100} * x = 56
4x/5 = 56
4x = 56*5
4x = 280
x = 280/4
=70
I rather prefer this way, but it's up to you :) I hoped this helped.