According to the given data we have the following:
customer purchases two bags of cherries $7.99 each bag, therefore,:
Total amount purchased by the client=two bags of cherries*$7.99
Total amount purchased by the client=$15.98
He pays with a $20 bill
Therefore, change will he recieve back=$20-$15.98
change will he recieve back=$4.02
If there is no tax, change will he recieve back is $4.02