The value of x is 11.
To find x, we must know how to calculate mean.
Mean = (all values added together) / (number of values in the set)
Now we can plug in our numbers:
8 = (7 + 4 + 10 + x) / 4
Using inverse operations, we will find that all the values must add up to 32 because 32 / 4 = 8.
(21 + x) / 4 = 8
32 - 21 = 11
Hence, x = 11.