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GMAT 1: 790 Q51 V51
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If the integer k is removed from the set of positive integers from 1 to n, inclusive, the average of the remaining integers is 25.8. What is the value of k?

A. 2
B. 11
C. 29
D. 36
E. 51

If the largest number n is removed, the remaining numbers are 1, 2, 3, ..., n - 1. Their average is (1 + (n - 1))/2 = n/2

If the smallest number 1 is removed, the remaining numbers are 2, 3, 4, ..., n. Their average is (2 + n)/2 = (n + 2)/2

Therefore:

n/2 ≤ 25.8 ≤ (n + 2)/2

n ≤ 51.6 ≤ n + 2

So n can be 50 or 51.

Since the sum of the remaining n - 1 integers is 25.8(n - 1), it must be an integer. Therefore, n - 1 must be divisible by 5. Hence, n = 51.

The sum from 1 to 51 is: 51 * 52/2 = 1326.

The sum after removing k is: 25.8 * 50 = 1290.

Therefore: k = 1326 - 1290 = 36.

Answer: D.

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Thank you for the prompt reply! I understand it now.
Bunuel
If the integer k is removed from the set of positive integers from 1 to n, inclusive, the average of the remaining integers is 25.8. What is the value of k?

A. 2
B. 11
C. 29
D. 36
E. 51

If the largest number n is removed, the remaining numbers are 1, 2, 3, ..., n - 1. Their average is (1 + (n - 1))/2 = n/2

If the smallest number 1 is removed, the remaining numbers are 2, 3, 4, ..., n. Their average is (2 + n)/2 = (n + 2)/2

Therefore:

n/2 ≤ 25.8 ≤ (n + 2)/2

n ≤ 51.6 ≤ n + 2

So n can be 50 or 51.

Since the sum of the remaining n - 1 integers is 25.8(n - 1), it must be an integer. Therefore, n - 1 must be divisible by 5. Hence, n = 51.

The sum from 1 to 51 is: 51 * 52/2 = 1326.

The sum after removing k is: 25.8 * 50 = 1290.

Therefore: k = 1326 - 1290 = 36.

Answer: D.

Radhika29
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