To find the number of books he bought we need to do this:
# of books bought at yard sale = # of books after yard sale - # of books before yard sale
# of books bought at yard sale: x (we don't know what is yet)
# of books after yard sale: 71 books
# of books before yard sale: 20 books
Now set up the equation to solve: x = 71 - 20
Solve the equation:
71 - 20 = 51
so x = 51
Dan bought 51 books at the yard sale.
