If p and n are positive integers, with p > n, what is the average of n

Let, n = 2
and p = 3

i.e. Average of 2 (n) consecutive multiples of 3 (p) starting from 3 (p) = (3+6)/2 = 9/2
and Average of 3 (p) consecutive multiples of 2 (n) starting from 2 (n) = (2+4+6)/3 = 12/3 = 4

The average of n consecutive multiples of p (starting with p), less the average of p consecutive multiples of n (starting with n) = (9/2)-4 = (1/2)

A. (p + n)/2 = (3+2)/2 = 7/2 INCORRECT
B. (p – n)/2 = (3-2)/2 = 1/2 CORRECT
C. p + n = (3+2) = 7 INCORRECT
D. p – n = (3-2) = 1 INCORRECT
E. p^2 – n^2 = 3^2 - 2^2 = 5 INCORRECT

Answer: option B

Hello bunuel
Please post the concept behind this question when you post the answer! I got the answer as b by picking numbers.

MANHATTAN GMAT OFFICIAL SOLUTION:

The easiest way to solve this problem is to pick convenient numbers for p and n. Say that p = 3 and n = 2, to follow the constraint that p > n.

Then we need to take the difference of two quantities:

1. The average of 2 consecutive multiples of 3 (starting with 3)
2. The average of 3 consecutive multiples of 2 (starting with 2).

The first quantity is the average of 3 and 6. That number is 4.5.

The second quantity is the average of 2, 4, and 6. That number is 4.

The difference between these two numbers is 0.5. We now look for the answer choice that yields 0.5 when p = 3 and n = 2. The only answer choice that does so is (p – n)/2.

Proving that this answer is correct in the abstract is significantly harder, but not outlandish.

The sum of n consecutive multiples of p = p + 2p + 3p + … + np.

Factor out a p to get p(1 + 2 + 3 + … + n). Now, the sum of n consecutive integers starting at 1 is the average of those integers times the number of integers.

The average of the integers is (n+ 1)/2. The number of integers is n.

So the sum of 1 through n is n(n + 1)/2. This is a formula that you shouldn’t memorize, but you should be distantly familiar with it, as passing acquaintances.

Thus, the sum of n consecutive multiples of p is pn(n + 1)/2, and the average of those n multiples is that sum divided by n, or p(n + 1)/2.

A similar argument gives you the average of p consecutive multiples of n. You can just switch p and n in the formula above: this average is n(p + 1)/2.

Subtracting these two gives you (p – n)/2.

The correct answer is B.

They are +ive Integers, 3 > 2

average of n consecutive multiples of p (starting with p) = (3+6)/2 = 4.5

average of p consecutive multiples of n (starting with n) => 2+4+6 / 3 = 4

Less than ?? => 4.5 - 4 = 0.5, we have to look for this now

3-2 / 2

Answer B

Solution:

We can let p = 3 and n = 2.

The average of 2 multiples of 3, starting with 3, is (3 + 6)/2 = 4.5.

The average of 3 multiples of 2, starting with 2, is (2 + 4 + 6)/3 = 4.

We see that the difference is 4.5 - 4 = 0.5, which is also the value of (3 - 2)/2. Therefore, B is the correct answer (note: none of the other choices will yield the value of 0.5).

Alternate Solution:

The average of n consecutive multiples of p, starting with p, is (p + 2p + 3p + … + np)/n = p(1 + 2 + … + n)/n = (p/n)(n(n + 1)/2) = p(n + 1)/2.

The average of p consecutive multiples of n, starting with n, is (n + 2n + 3n + … + pn)/p = n(1 + 2 + … + p)/p = (n/p)(p(p + 1)/2) = n(p + 1)/2.

Thus, subtracting the second expression from the first, we get p(n + 1)/2 - n(p + 1)/2 = (np + p - np - n)/2 = (p - n)/2.

Answer: B

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