10 is the answer. You know that there are 4 numbers and you have to add the numbers then divide them by four to get the mean. So all you have to do is reverse the steps, multiply 30 by 4 to get 120, then add 12+50+48 to get 110. Then subtract, 120-110=10. To check if it is correct add 12, 50, 48, and 10 to get 120, then divide by 4 to get 30. And voila, there is the answer. I hope this helped for future problems like this one.
