Question

instructions
choose the best answer. if necessary, use the paper you were given.
question
the average of four numbers is 15. if the sum of three of the numbers is 50, what is the fourth number?
10
15
20
25

Explanation:

Step1: Calculate the total sum of four numbers

The formula for the average $\bar{x}=\frac{\sum_{i = 1}^{n}x_{i}}{n}$. Given $n = 4$ and $\bar{x}=15$, then the sum of four numbers $S_{4}=n\times\bar{x}=4\times15 = 60$.

Step2: Find the fourth number

Let the fourth number be $x$. We know that the sum of three numbers $S_{3} = 50$. And $S_{4}=S_{3}+x$. So $x=S_{4}-S_{3}$. Substitute $S_{4} = 60$ and $S_{3}=50$ into the formula, we get $x=60 - 50=10$.

Answer:

10