Question

the arithmetic mean of a set of nine numbers is 5. when one more number is added to the set, the new mean is 5.5. What number was added?

Answer 1

I answered that already.
The total for 9 numbers is 45 and the total for 10 numbers is 55. Take the difference.

Answer 2

(9(5)+x)/10=5.5
45+x=55
x=10