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

**A.**14**B.**16**C.**18**D.**20

Answer: **C.** 18

Let’s denote the four numbers as * a*,

*,*

**b****, and**

*c**.*

**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**.