Probably/Coin problem? (20 Characters)

I have absolutely no idea how to do this problem, so if you understand it, can you please explain how I should do it? I've played around with it, but I don't really understand.

How much money do you have if:
1. you only have nickels, dimes, and pennies;
2. 3/7 of the coins are pennies;
3. 20% of the coins are dimes; and
4. 13 of them are nickels

Let T = total number of coins

Then: Number of pennies = (3/7)T, number of dimes = (1/5)T, and of course we know that there are 13 nickels

Now, since (number of pennies) + (number of dimes) + (number of nickels) = T,

Then: (3/7)T + (1/5)T + 13 = T => 13 = T - (3/7)T - (1/5)T

Combining the terms on the R.H.S. gives us: 13 = (13/35)T
So: Solving for T we get that there are 35 coins, which means we have (3/7)*35 = 15 pennies, (1/5)*35 = 7 dimes, and 13 nickels for a total of $1.50.