Dimes are worth twice as much as a nickel, therefore you would divide the amount of nickels by half:
80 / 2 = 40
Therefore you have 40 dimes.
Another way to do this is to find your current amount of money, since dimes are worth 10 cents and nickels are worth 5 cents:
80 * 5 = 400 cents
400 / 10 = 40 dimes
