The mean of a set of numbers is calculated by adding them all together and then dividing the sum by the amount of values in the data set. In this scenario, we know the mean is 30, and there are 3 numbers in the data set, 10, 20, and an unknown number (x).
Let's set up an equation with the information we have.
30 = (10 + 20 + x) / 3
Now we can multiply both sides by 3 ==>
90 = 10 + 20 + x
Combine like terms ==>
90 = 30 + x
Subtract 30 from each side ==>
60 = x
And now you have your answer!
The value of x = 60 would make the mean of the data set 30.