I am assuming that there is a typographical error in inputting this problem.
3535 cents is 35 cents
55 cent cons is 5 cent coins
1010 cent coins is 10 cent coins
2525 cent coins is 25 cent coins
So, Paul can pay Paula in any combination.
1) 7 x 0.05 = 0.35
2) (5 x 0.05) + (1 x 0.10) = 0.25 + 0.10 = 0.35
3) (3 x 0.05) + (2 x 0.10) = 0.15 + 0.20 = 0.35
4) (1 x 0.05) + (3 x 0.10) = 0.05 + 0.30 = 0.35
5) (2 x 0.05) + (1 x 0.25) = 0.10 + 0.25 = 0.35
6) (1 x 0.10) + (1 x 0.25) = 0.10 + 0.25 = 0.35
The difference between the largest and the smallest number of coins he can use to pay her is.
1) 0 = all coins are 5 cents
2), 3) and 4) 0.10 - 0.05 = 0.05
5) 0.25 - 0.05 = 0.20
6) 0.25 - 0.10 = 0.15
