Answer: c-d=n
Explanation: Suppose I had 30 cents. I knew I only have nickels and dimes, and I know I have 2 dimes, that means I have 20 cents in dimes. so 30 cents (c) - 20 cents (d) = 10 cents (n), and 10/5 = 2, so I would have 2 nickels. I hope that explains it, if not let me know.