A and B share a $21 pizza. B eats 2/3 as much as A. How much should they each pay. Hint: There is still some pizza left.
3/3 + 2/3 = 5/3
Divide $21 into 5 parts, and A pays for 3 and B pays for 2.
21/5 = 4.20
A pays 3*4.2 = 12.60
B pays 2*4.2 = 8.40
total: $21.00