Three out of four numbers have a sum of 30. If the average of the four numbers is 12, what is the fourth number?

Answer: C. 18

Let’s denote the four numbers as a, b, c, and d.

We are given that three of the numbers have a sum of 30:

a+b+c = 30

And we are also given that the average of the four numbers is 12:

(a+b+c+d)/4 = 12

=> a+b+c+d = 48

Now, substituting a+b+c=30 into this equation gives us:

30+d = 48

Subtracting 30 from both sides gives us:

d = 48−30

d = 18

So, the fourth number is d = 18.