The mean of four numbers is 30 are 12,50 and 48 what is the fourth number. a)129 b) 10 c) 12 d)30

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.