pls explain

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melon: Senior | Next Rank: 100 Posts; Posts: 34; Joined: Wed Jan 05, 2011 9:15 pmFollowed by: 1 member; Thanked: 3 times

by melon » Fri Jul 15, 2011 9:31 pm

Sequence A is defined by the equation An = 3n + 7, where n is an integer greater than or equal to 1. If set B is comprised of the first x terms of sequence A, what is the median of set B ?

(1) The sum of the terms in set B is 275.

(2) The range of the terms in set B is 30.

I understood why 1 is sufficient but the Ans is D (meaning each statement alone is enough)
I very well know tht for consecutive intergers mean is equal to median
here he says range is 30 and n greater than or equal to 1
so n can be 1 or 2 or 3...etc or rather can start from 1 or 2 or 3.
so when n is unknown how can we calculate median

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Source: Beat The GMAT — Data Sufficiency | Fri Jul 15, 2011 9:31 pm

newgmattest: Senior | Next Rank: 100 Posts; Posts: 52; Joined: Wed May 18, 2011 8:48 pm; Thanked: 4 times

by newgmattest » Fri Jul 15, 2011 9:43 pm

Elements in A start from 10,13,16,19...

1) Sum is 275, so we can extend the terms in A to have the total as 275.
We would know the number of terms as well as the terms, so we can find the median of set B.
Sufficient.

2) The lowest term in B is 10. Since range is 30 the highest term is 40. So B consists of 10,13,16,19,22,25,...40. Again, we can find the median of B.
Sufficient.

Hence D

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gmat25: Master | Next Rank: 500 Posts; Posts: 189; Joined: Wed Jul 06, 2011 6:57 amFollowed by: 1 member; Thanked: 17 times

by gmat25 » Fri Jul 15, 2011 11:26 pm

melon wrote:Sequence A is defined by the equation An = 3n + 7, where n is an integer greater than or equal to 1. If set B is comprised of the first x terms of sequence A, what is the median of set B ?

(1) The sum of the terms in set B is 275.

(2) The range of the terms in set B is 30.

I understood why 1 is sufficient but the Ans is D (meaning each statement alone is enough)
I very well know tht for consecutive intergers mean is equal to median
here he says range is 30 and n greater than or equal to 1
so n can be 1 or 2 or 3...etc or rather can start from 1 or 2 or 3.
so when n is unknown how can we calculate median

See u can look at this problem in this way...if u know what is the specific value of X then u can calculate median right...because u already have the eq'n given in the question though which u can find the values of the series. so all u need is X.

Op B:- first term = 3*1 + 7 = 10

now if there are X terms then the last term should be 3*x + 7 = 3x + 7 right

range = last term - first term

=> 3x + 7 - 10 = 30 (range is given in Op B), so u get X and hence u can find median.

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