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.
The length of the altitude from the right angle to the hypotenuse of a right triangle is the <u>geometric</u> mean of the two segments that make the hypotenuse.
