Logical Reasoning - Number Series - Discussion

Look carefully for the pattern, and then choose which pair of numbers comes next.

2. 

8 11 21 15 18 21 22

[A]. 25 18
[B]. 25 21
[C]. 25 29
[D]. 24 21
[E]. 22 26

Answer: Option B

Explanation:

This is an alternating addition series, with a random number, 21, interpolated as every third number. The addition series alternates between adding 3 and adding 4. The number 21 appears after each number arrived at by adding 3.

Deepanshu said: (Nov 24, 2010)  
I do not seem to get this one.
Please explain in layman

Monica said: (Nov 27, 2010)  
Can you explain the addition series in detail? please.

Durai said: (Dec 16, 2010)  
This is simply confusing. I think, this question is wrong.

Manas said: (Jan 13, 2011)  
Kindly explain it elaborately, I could not understand the solution.

Saurabh said: (Jan 16, 2011)  
Hi,

Random number is 21 and is repeated after two number.

3 is adding to 22 the result is 25 and 21 is repeated again.

Sankari said: (Feb 11, 2011)  
8+3 = 11
11+4 = 15
15+3 = 18
18+4 = 22
From above we can conclude as difference b/w first two no.is 3.
Third no. is interpolation, it is repeated as every third no.
so next pair is 25 21 (22+3 =25)

Sriharsha said: (Feb 15, 2011)  
What is interpolation?

G.Priyanga said: (Feb 17, 2011)  
I did not get this answer. Please explain it clearly.

Prathyusha said: (Feb 22, 2011)  
11-8=3
15-18=3
22-25=3
so 25 21 is answer

Sameh said: (Mar 3, 2011)  
I'm not convinced , if the answer was only 25 it would be ok but 25 21 is confusing..

Siva said: (Mar 5, 2011)  
What is your confusion boss it is easy to get because hare to first number first 3 is added and then as third number interpolation would come and then what is there before interpolation number that would be added by 4 like this it will be go on.

Sundar said: (Mar 22, 2011)  
8 11 [21] 15 18 [21] 22 25 [21] 29 32 [21] 36 39 [21] 43 46 [21] ....

Here 3, 4 is added alternatively and [21] injected as given below:

Inital = 8
8 + 3 = 11
********** [21]
11 + 4 = 15
15 + 3 = 18
********** [21]
18 + 4 = 22
22 + 3 = 25 <----- Required Answer
********** [21] <----- Required Answer
25 + 4 = 29
29 + 3 = 32
.........etc

Priya said: (Mar 31, 2011)  
Among all the above solution, I think Sundar is right.

Azhar said: (Apr 3, 2011)  
Good explained by Sundar.

Bharat said: (May 9, 2011)  
Thanks Sundar.

Vipin said: (Jun 10, 2011)  
Great job sundar.

Navjot said: (Jul 9, 2011)  
8,11,[21],15,18,[21],22,25,[21],29,32,[21]

1st no is 8,when 7 is added ,it gives 15 (4th number in series)
2nd no is 11 ,again when 7 is added,it gives 18(5th no in series)
21 is interpolated as every 3rd no.


4th no is 15 ,when 7 is added,it gives 22 (7th no)
5th no is 18,when 7 is added,it gives 25 (8th no)
again 21 is interpolated as 3rd no (here its 6th).


7th no is 22 ,when 7 is added,it gives 29 (10th no)
8th no is 25,when 7 is added,it gives 32 (11th no)
again 21 is interpolated as 3rd no (here its 9th).and so on.....

Shankari said: (Sep 12, 2011)  
Really great, I can understand sundar's explanation thank you.

Sudhakar said: (Nov 12, 2011)  
Actually it would be 25, 29. Adding 3 and alternate adding 4. So it should come 25, 29.

Simarpreet Kaur said: (Nov 18, 2011)  
I am not able to understand it please explain it as a layman could understand.

Priyanka said: (Nov 21, 2011)  
SOLUTION:
Here number 21 put as a interpolatn dat means only to confuse you. So just excluding that series will be like as:
8+3=11 (add 3 & interpolation 21)
15+3=18 (add 3 & interpolation 21)
Now 22+3 =25 (add 3 & interpolation 21)

So answer wil be B) 25 21 because 25 final anwer and 21 as a interpolation.

Hope now you get it.

Saikanth said: (Mar 4, 2012)  
8+3 = 11
11+4 = 15
15+3 = 18
18+4 = 22
From above we can conclude as difference b/w first two no.is 3.
Third no. is interpolation, it is repeated as every third no.
so next pair is 25 21 (22+3 =25)

Paramjeetkaur said: (Mar 10, 2012)  
Thanks Sundar.

Saravanan.O said: (May 25, 2012)  
There are two condition is here

1. Every 3rd number is 21
2. Difference between 2 and 1 number is 3.

In this way,
8-11=3
21
15-18=3
21
22-25=3. so the answer is 25 21

Harpreet Singh said: (Oct 21, 2013)  
21 is common after every 2 number. So

8 11 21 15 18 21 22...

8+3 = 11 21.
15+3 = 18 21.
22+3 = 25 21.

So answer is 25 21.

Ashish Minz said: (Mar 7, 2014)  
@Harpreet Singh.

How he get 3..

11-8 = 3 21 is fixed.
18-15 = 3.

Hasan Faraz said: (Apr 24, 2014)  
Good question.

8 11 21 15 18 21 22
It is a sequence of adding 3, putting 21, adding 4 to the result of adding 3.

Ashok Bijoy said: (May 5, 2014)  
8(+3) = 11, 21.
15(+3 = 18, 21.
22(+3) = 25, 21.

Dharmishtha said: (Jul 1, 2014)  
(8+3)=11 & 21 is common in this series.
(15+3)=18 & 21 is common in this series.
(22+3)=25 & 21 is common in this series.

So, 8, 11, (21), 15, 18, (21), 22, 25, (21),...

Jeyanthi said: (Oct 9, 2014)  
This type of question is easy but clearly understand the logic.

Ganesh said: (Nov 10, 2014)  
8+3 = 11

15+3 = 18

22+3 = 25

So 21 is common in this series.

Zakariya said: (Jun 10, 2015)  
I don't understand this type questions.

G.N.Lakshmi said: (Jan 16, 2016)  
8+3 = 11.
15+3 = 18.
22+3 = 25.

21 is median.

K Devraju said: (Mar 14, 2016)  
21 is Common in this Series.

So you can See,

=> 8 + 3 = 11.
=> 15 + 3 = 18.
=> 22 + 3 = 25.

Answer comes (25 21).

Kapil Kapoor said: (Jun 15, 2016)  
@Sundar,

Thanks for the wonderful explanation.

Subhash said: (Jun 27, 2016)  
@ Sundar

Thank you, your explanation is very good.

Shridhar said: (Aug 20, 2016)  
@Sundar, good answer but please tell me in a simple way.

Rama Kant said: (Sep 26, 2016)  
8 + 7 = 15,
15 + 7 = 22,
11 + 7 = 18,
18 + 7 = 25,
21 is constant.

Every number add 7 except 21.

Joshua said: (Oct 22, 2016)  
I don't understand. Please help me.

K.Chandra Babu said: (Oct 28, 2016)  
Leaving 21 as a third number.

8 + 3 = 11,
15 + 3 = 18,
22 + 3 = 25 so answer will be 25 21, and also first number difference is 7.

Divakar .U said: (Nov 21, 2016)  
8 11 {21} = The difference is 3.

15 18 {21} = The difference is 3.

22 25 {21} = The difference is 3.

Swapneswar said: (Jan 6, 2017)  
Good explanation @Sundar.

Maranna T said: (Mar 25, 2017)  
Thanks for all the given explanation friends.

Salman Khan said: (Apr 19, 2017)  
8 11 21 15 18 21 22 25 21

8 + 3 = 11
21
15 + 3 = 18
21
22 + 3 = 25
22.

Mehtab said: (May 1, 2017)  
There is a mistake in explanation. 21 is repeating as random number while 7 is added to each pair to get the new pair.

Annam Kanaka Nishanth said: (Sep 6, 2017)  
11.21-8=3.21
15-11.21=4.21
18.21-15=3.21
22-18.21=4.21
Hence
22+3.21=25 21.

Soham Shinde said: (Sep 8, 2017)  
Nice explanation @Saurabh.

Pradip Paul said: (Feb 18, 2018)  
8+3=1121.
15+3=1821.
22+3=2521.

Ans is 2521 (B).

Priya Kumari said: (May 8, 2019)  
Hi, I am not understanding this, Please anyone help me.

Prajwal said: (Jul 2, 2019)  
Thanks for all for explaining it.

Dinesh said: (Jul 4, 2019)  
Thanks for explaining the answer @Sundar.

Srikanth Singh said: (Aug 2, 2019)  
Thanks @Sundar.

Sarveshwar said: (Aug 20, 2019)  
8+3=11,
11-21=10
10+3=13.

15-18=3,
18-21=3
3+3=6.

Then 13-6=7.
22+3=25, 25-4 = 21.
22,25,21.

Sushil said: (Apr 18, 2020)  
25 29 is the correct answer.

Anshu said: (Feb 11, 2021)  
@Sundar and @Navjot.

Good explanation, Thanks.

Fama said: (Aug 13, 2021)  
2529 is the correct answer.

Post your comments here:

Name *:

Email   : (optional)

» Your comments will be displayed only after manual approval.