Multiple choice

In a set of three numbers, the average of first two numbers is 21, the average of the last two numbers is 24, and the average of the first and the last numbers is 15. What is the average of the three numbers?

  1. 20

  2. 60

  3. 25

  4. 18
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let the numbers be x, y, z. (x+y)/2 = 21 => x+y=42. (y+z)/2 = 24 => y+z=48. (x+z)/2 = 15 => x+z=30. Adding these: 2(x+y+z) = 42+48+30 = 120, so x+y+z = 60. The average is (x+y+z)/3 = 60/3 = 20.

AI explanation

Using the formula Sum = Average x Count, the sum of the first two numbers is 21 x 2 = 42, the sum of the last two is 24 x 2 = 48, and the sum of the first and last is 15 x 2 = 30. Adding these three sums gives 42 + 48 + 30 = 120, which equals twice the sum of all three numbers. The sum of all three numbers is 120 / 2 = 60, making their average 60 / 3 = 20.

Keep practicing — related questions