The mean of 4 test scores is 90. what is the sum of the scores?

360. 
A general rule of thumb when dealing with questions like these is using the following equation:

(number of numbers) x (mean) = sum of numbers

Proof/Example: 7, 10, 13
There are 3 numbers here and the mean is 10. Multiply them together and you will get 30, just as you will get 30 from adding them together. 

In this case, there are 4 numbers and the mean is 90, so 4 x 90 = 360.

To find a mean of a number or the average  you first would add all the test scores up. then divide by the number of the quantity there are.
* the quantity is 4
so before you divide by the quantity you already added the sum of the test so if you do the problem backward and instead of dividing by 4 you would multiply by 4. 

So 90 times 4 which is 360.
Thus the sum of the score is 360.