To find the total amount Dave and Mel paid, apply the taxes to their initial purchases.
Dave paid $15 for a baseball. There was an 8% tax added to this total afterwards. To add the tax to the amount Dave paid, convert the percentage into a decimal:

Multiply the cost of the baseball by this decimal to find the tax:

Add the tax onto the cost:

Dave paid $16.20 in total for the baseball.
Mel paid $20 for a football. There is an 8% tax added to this total afterwards. We already have the decimal for an 8% tax, so we can multiply the football's cost by the decimal:

Add the tax onto the cost:

Mel paid $21.60 for the football.
Subtract the bigger cost by the smaller cost:

There is a
$5.40 difference between Dave's and Mel's purchases.