Question

the mean of four numbers is 30. if three of the numbers are 12, 50, and 48, what is the fourth number?
A.120
B.10
C.12
D.30

Answer 1

for example, n is the number we want to find

The sum of the four numbers divided by 4 is equal to the mean.
the sum/4 = mean
(12 + 50 + 48 + n)/4 = 30
(110 + n)/4 = 30
(110 + n) = 30 × 4
(110 + n) = 120
n = 120 - 110
n = 10

The fourth number is 10