As a general rule of thumb, to find the mean of a given set of numbers:
add up all the numbers youve been given and divide by the number of values you have.
e.g.
Find the mean of these values: 5, 10, 8, 9, 20.
so add them up: 5+10+8+9+20 = 52
and divide by the number of values : 52/5 = 10.4
if your mean value has to be one of the given values and not somewhere inbetween then round to the nearest, here it would be 10.