Question

The mean of 7 numbers is 15. When an 8th number is added the mean decreases to 12. What is the 8th number?

Answer 1

The 8th number is -9

Explanation:

With an average, the sum of the seven numbers divided by 7 is 15.

In order to add an 8th number, we set up

`(7*15+x)/(8)=12\\105 + x = 96\\x = -9\\\\(105-9)/(8)=(96)/(8)=12`