Answer:
6
Step-by-step explanation:
The average of a set of numbers is their sum divided by their number. This fact lets us write an equation to find the missing number.
Setup
Let x represent the missing number. The average of the 4 numbers is ...
(x +18 +11 +25)/4 = 15
Solution
Multiplying by 4 gives ...
x +54 = 60
Subtracting 54, we find the missing number:
x = 6
The other number is 6.
