Allowance Formula

My daughter and I have been trying to come up with a fair method to determine how much allowance to pay each child. I wanted something that would reward good behavior, punish bad behavior, and recognize that as children get older, they need more money (a painful fact of life for parents). What I came up with is this:

his or her age
times the number of A's on his or her last report card
times 1.5
divided by (the number of times he or she was punished last month + 2)

or more plainly: 1.5 * age * grades / (punished + 2)

So the formula would be: 1.5ag / (p + 2)

Which means, for example, if you're 15, you got 6 A's on your last report card, and have not been in trouble, you'd get $67.50 a month ($16 a week). If you're 14, got 3 A's, and were punished twice in the last month, you would only get $15.75 for the month ($4 a week).

Obviously, the multipliers can be adjusted to suit any economic status or expectation, but I think that by and large, this formula will do the trick.