Well, given that a quarter equals 5 nickels, we can reason that, in order to get 12 coins, we would have to get 10 nickels (equals 50 cents) and 2 quarters (the other 50 cents, to make a dollar).
You didn't say whether you needed this expressed in a mathematical formula, but knowing the answer already, it shouldn't be hard to come up with one if necessary.
Hope this helps.
0.015 or 3/200
15 divided by 1000 is 0.015
15/5 is 3 1000/5 is 200
12/25 that will be the answer so its the percent time always helps.
