You can do this easily by dividing the value of each coin and dividing it by the value of a dollar and multiply it by 100%. But before you do that, you need to first make them the same unit.
Nickel = 5 cents
Dime = 10 cents
Dollar = 100 cents
Nickel:
0.05 x 100% = 5%
Dime:
0.10 x 100% = 10%
But if you are looking for the percentage of both a nickel and a dime together, just add them up and you'll get 15%.
I did say the answer is $35. Because, you should use the highest factor or whatever operation the problem is using, to find the answer. Not sure if it's some two-step problem....