The average of four numbers is 25. If three of the numbers are 20, 30, and 40, what is the fourth number?

A. 10

B. 25

C. 20

D. 15

The fourth number is calculated to be 10, based on the given average of 25 for the four numbers. Thus, the answer is A. 10.
;

Answered by Anonymous | 2025-07-11

To find the fourth number, we need to first understand what it means for the average of four numbers to be 25.
The average is calculated by dividing the sum of all the numbers by the total count of the numbers. In this case, we have four numbers. Thus, we can write the equation for average as:
Average = Total number of numbers Sum of all numbers ​
Given that the average is 25, we have:
25 = 4 20 + 30 + 40 + x ​
Here, x represents the fourth number. Now, we need to solve this equation to find x .
First, calculate the sum of the known numbers:
20 + 30 + 40 = 90
Substitute this sum back into the equation:
25 = 4 90 + x ​
To clear the fraction, multiply both sides of the equation by 4:
25 × 4 = 90 + x
100 = 90 + x
Next, solve for x by subtracting 90 from both sides:
x = 100 − 90
x = 10
Therefore, the fourth number is 10 .
The correct multiple-choice option is a. 10 .

Answered by LiamAlexanderSmith | 2025-07-21