How do you find the missing number when the average is given?

Suppose you have 5 numbers: 10, 15, 20, 25, 30
The average is 20.
Now, if we know the average, the number of observations which are 5 on this example,
Then, the missing number is 5*20 - (10+20+25+30) = 100-85 = 15

More generally, the missing number is the mean times the number of observations minus the numbers that you know.