Kim orders a $10 meal and a drink for 2.5.  She gives a 20% tip.  She pays with a $20 bill.  How much does she get in change?
The dinner cost 10+2.5=$<T>add(10, 2.5)=12.5.
The tip comes to 12.5*.2=$2.5.
So the total cost is 12.5+2.5=$<T>add(12.5, 2.5)=15.0
That means she gets 20-15=$<T>subtract(20, 15)=5 in change.
