If five numbers have a mean of -10 it means they'll have a sum of 5(-10) = -50.
That's because the mean is the sum divided by the count, so the sum is the mean times the count.
Here our sum is -10 + 0 + -5 + -15 = -30, so our fifth number must be -50 - -30 = -20 to make the sum -50 and the mean -10.
Answer: -20
