Question

instructions
choose the best answer. if necessary, use the paper you were given.
question
the mean of four numbers is 20. if one of the numbers is 15, what is the sum of the other three numbers?
25
45
65
85

Explanation:

Step1: Find the total sum of the four numbers

The formula for the mean $\bar{x}=\frac{\sum_{i = 1}^{n}x_{i}}{n}$. Given $n = 4$ and $\bar{x}=20$, then $\sum_{i=1}^{4}x_{i}=n\times\bar{x}=4\times20 = 80$.

Step2: Find the sum of the other three numbers

Let the four numbers be $a,b,c,d$. We know one number, say $a = 15$, and $\sum_{i = 1}^{4}x_{i}=a + b + c + d=80$. Then $b + c + d=\sum_{i = 1}^{4}x_{i}-a=80 - 15=65$.

Answer:

65