a change -15 can be obtained from the following situation.
Assume you have a debt of $100, if you have to pay $30 of your debt, each two days, the rate at which you pay your debt per day is obtained by calculating the quotient between 30 and 2. Furthermore, consider the money you pay as a decrese in your total money, then 30 is takes as negative quantity.
Then, you have:
-30/2 = -15
Hence, the rate of change of your total money is -15 per day