mean = average
Find the sum of the five numbers and divide by five....
You can do the simple calculation.
In an example like this, where the numbers are close together and there are only a few of them, you can also find the average by guessing an average and finding how the given numbers compare to that average.
For example, suppose you look at these numbers and think the average is about 85. Now compare each of the numbers to that guess:
84: -1
86: +1
85: 0
76: -9
89: +4
-------
sum: -5
The sum of the five numbers together is 5 less than what it would be if the average were 85, so on average the numbers are 1 less than 85. So the average of the five numbers is 84.