Equation: $5.50*x=$108.50
⇒ ($5.50/$5.50)*x= $108.50/ $5.50
⇒ x= 19.7
Because Beth cannot buy 19.7 shirts (the number of shirts has to be a real number) and obviously Beth cannot buy 20 shirts (because she does not have enough money), the number of shirts she can buy is 19.
Hope this would help~