Sequence A is defined by the equation An = 3n + 2, where n is an integ

Question

Sequence A is defined by the equation  A_n = 3n + 2 , 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 220.

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

fluke · Accepted Answer

A_n=3n+2 
 A_1=3*1+2=5 
 A_2=3*2+2=8 
 A_3=3*3+2=11 
.
.
.
As we can see that the set
A={5,8,11,14,17,20,23,26,29,32,35,38..................}
B={5,...}

We just need to know the number of terms to get the median.

1.
Sum of an arithmetic series:
 S_n=\frac{n}{2}  
We know,
 a_1=5 
 d=3 
Solving, we get n=11 and the median.
Sufficient.

2.
Range is known:
Min=5, we know
Max=35, as the range=30
Thus, B={5,8,11,14,17,20,23,26,29,32,35}
Median=20
Sufficient.

Ans: "D"

IanStewart · Answer

Fluke's solution is great, but you don't need to go through all the work. If we see that the sequence is

5, 8, 11, 14, 17, ....

then certainly the more terms we have, the bigger the sum will be. So Statement 1 simply has to be sufficient: there can only be one number of terms that will give a sum of exactly 220, and if we know the number of terms, we can find the median. Similarly, the more terms we have, the bigger the range will be, so there can only be one number of terms that gives a range of exactly 30. The answer is D.

agdimple333 · Answer

What if i take min = 11 and as range = 30 implies B = 41
so set = {11,14,17,20,23,26,29,32,35,38,41}
than the median = 26

what i am confused here is that in such kind of Q, are they asking if we can find the median based on the given information or not. 
Or 
are we suppose to determine that with given info there exists only single median (you know what i mean)?

But now when i see statement 1, according to which also we can construct different sets, just that the sum of all elements should be 220. So basically i think the Q is asking if we can find the median based on the given info or not.

fluke · Answer

Based on the given info, you can just create ONE set:
set B can't start from 11.
We know 
A={5,8,11,14....}
AND it is given in the stem that:
"If set B is comprised of the   first x terms   of sequence A".
Thus, B can't start from 11.

And now, when a series is starting with 5, we know this for sure. We also know the summation of the series from st1. We will know the exact set to form. Once we know the set, we can find the median.

Did I answer your question?

sudhir18n · Answer

one thing we must remember is in DS the 2 statements can never contradict each other.

If your statement 2 stats with 5..................41
then the st.1 should mention the sum of set B as 226 and not 220.

Hope that helps.

Spidy001 · Answer

Sequence is like 5 8 .....

1. Sufficient

sum of terms is 220

so enough to find the last term and all the terms upto the last term (as the series is in A.P.)

=> enough to find median

2. Sufficient

from the range and the first term , we can find the last term and all the terms upto the last term (as the series is in A.P.)

=> enough to find median

Answer is D.

amit2k9 · Answer

a1=5,a2=8,a3=11 and so on.

a gives 120 = n/2  where d = 3 and a = 5 here.

thus the xth element can be found out. Hence the median.

Sufficient.

b gives a = 5 and xth element = 35.
thus median can be found out for AM.
Sufficient.

D it is.

sam26can · Answer

Hi Bunnel and Fluke,

The problem reads that Sequence A is defined by the equation An = 3n + 2, therefore Sequence A is definitely in Arithmetic progression.

Then it reads If set B is comprised of the first x terms of sequence A- How can we assume from this that set B in Arithmetic progression.

As per Fluke's solution, statement A is only sufficient if it is assumed that set B is in AP.

From statement B we know that the t1= 5 and tn=35

set B could be = {5, t2, t3, t4, ...t(n-1),35}

But if we combine both the information then

Assuming that set B is not in AP

5+t2+t3+t4+...+t(n-1)+35=220
t2+t3+t4+...+t(n-1)=180

Therefore set B could be= (5,30,30,30,30,30,30,35)  , median 30
or set B could be=(5,24,28,29,31,32,36), median 30
or set B could be= (5,7,16,18,20,22,27,30,35), median 20

The way I understand the problem, if we assume that set B is in AP then the answer choice is D
If we assume that set B is not in AP and can consist of both different or same integers then the answer choice is E

It would be very helpful if you can please provide some insight on my understanding.

Bunuel · Answer

Set A = {5, 8, 11, 14, 17, 20, ...}, an arithmetic progression as you noted.

Set B is comprised of  the  first  x terms  of set A, so set B must also be an arithmetic progression. For example, if x=3, so if set B is comprised of  the  first  3 terms  of set A, then set B = {5, 8, 11}.

Does this make sense?

Aboyhasnoname · Answer

Bunuel   bb

One question here, Why are we assuming that Sequence A starts with n=1? ....Sequence is defined by the equation given and n is the integer greater than equal to 1? 
It can start from n = 3 also....Shouldn't the question specify this explicitly.

Bunuel · Answer

Do not overcomplicate and overthink. That’s very counterproductive. We are told that “Sequence A is defined by the equation  A_n = 3n + 2 , where n is an integer greater than or equal to 1,” which means the sequence starts at n = 1 and the formula applies to all n greater than or equal to 1 .

suntnon · Answer

how is d = 5? should it not be 3

Bunuel · Answer

That's obviously a typo in fluke's solution. Fixed now.

Sequence A is defined by the equation An = 3n + 2, where n is an integ

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Sequence A is defined by the equation An = 3n + 2, where n is an integ

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