There are five numbers, the average of which is 7. If you change one of them to 9, the average of your five numbers is 8. What is the original number?

There are five numbers, the average of which is 7. If you change one of them to 9, the average of your five numbers is 8. What is the original number?

The original sum is 5 × 7 = 35
The new sum is 5 × 8 = 40
It's increased by 5, so the original number is 9-5 = 4