Multiple choice

The average of some natural numbers is 15. If 30 is added to the first number and 5 is subtracted from the last number, the average becomes 17.5. Find the number of natural numbers.

  1. 15

  2. 30

  3. 20

  4. 10

Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let n be the count. Original sum is 15n. After modifications, sum becomes 15n + 30 - 5 = 15n + 25. New average: (15n+25)/n = 17.5. Solving: 15n + 25 = 17.5n, giving 25 = 2.5n, so n = 10.