Word Problem 5th Ed. Qu# 6 pg 137

[bmshah1]

This DS problem is asking if we can determine the sum of the elements in a Set S (S is a set of consecutive integers). The first condition only says the largest element in S is 55. So I can easily construct a set S with just two consecutive numbers with 55 as one of them and determine the sum. This would be true as well for Statement# 2 where I can construct a set with 11 elements and easily calculate the sum. By arguing this way the correct answer would be D in my opinion. Why is then the official answer talking about calculating the average of the set S and not the sum and provides C as the answer. I can see why C would be a logical choice if I had to calculate the average and not the sum. Any ideas.

[divineacclivity]

hey, can you please post the whole question?
From the little I get from your question above, I can only tell the following few things that you'd find useful while solving such questions:
1. (average) x (No. of items in the set) = sum
2. For a series of consecutive numbers, following is one of the various ways to calculate sum:
1, 2, 3, ...., 55
Average = (first element + last element)/2 = (1+55)/2 = 28
Sum = average x number of elements = 28 x 55 = 1540
3. Sum of a series can also be calculated as = n x (n+1)/2
= 55 x 56/2
= 55 x 28
= 1540
Hope this helps.

[bmshah1]

hey, can you please post the whole question?
From the little I get from your question above, I can only tell the following few things that you'd find useful while solving such questions:
1. (average) x (No. of items in the set) = sum
2. For a series of consecutive numbers, following is one of the various ways to calculate sum:
1, 2, 3, ...., 55
Average = (first element + last element)/2 = (1+55)/2 = 28
Sum = average x number of elements = 28 x 55 = 1540
3. Sum of a series can also be calculated as = n x (n+1)/2
= 55 x 56/2
= 55 x 28
= 1540
Hope this helps.

Thx for the reply. I found the fault with my logic. For yr ref here is the full problem St.

If s is a set of consecutive integers what is the sum of the elements in s

1 the largest element in s is 55
2 there are 11 elements in s

[divineacclivity]

:)

[jlucero]

Good work, GMAT forum helpers!

[divineacclivity]

that's kind of you J. thanks.

[tim]

:)