The sum of 2 numbers is 9 what is the 3rd number. The average of the 3 numbers is 10

Answer 1

Let the three numbers be x, y, and z. So z is the third number.

The sum of two numbers is 9: x + y = 9

The average of the three numbers is 10:

(x+y+z)/3 = 10

x + y + z = 30

9 + z = 30

z = 21