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

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

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Not sure what the question is but i hope this x = years100 * x