nitesh50
Hi
VeritasKarishmaI had a query wrt your articles on the made easy series.
Q. IN how many ways can A,B,C,D,E,F be arranged in a circular table provided that A cannot sit next to D or E?
This question is a slight variation in comparison to your question on the Combinations Article 3: Circular Arrangements.
I recon that it is slightly complicated.
Here was my approach:
If only
1. D/E is selected: 2 ways * 6 ways( 3c2 ways of choosing people to be beside A and 2! ways of arranging them) * 2 ways( the other people arrangement)
2. D and E both are selected:
3c2(out of remaining 3 people, 2 are selected)* 2 ways (those 2 people are arranged) * 2 (D,E are arranged in remaining places)
3. A is not selected: 4!
SO total possibilities:
24+ 12+24= 60 ways.
Am i correct in my reasoning ?
and is there any simpler and faster way to do this question?
Looking forward to your reply!
Regards
Nitesh
Hey Nitesh,
I am not sure what your question is. There are two variations possible:
Q. IN how many ways can A,B,C,D,E,F be arranged in a circular table provided that A cannot sit next to D and E at the same time. (Implying that A sitting next to D if E is far away is ok)
or
Q. IN how many ways can A,B,C,D,E,F be arranged in a circular table provided that A can sit next to neither D nor E? (Implying that both D and E should be far away from A)
The first question is discussed as question 2 in my post:
Question 2: There are 6 people, A, B, C, D, E and F. They have to sit around a circular table such that A cannot sit next to D and F at the same time. How many such arrangements are possible?
Solution: Total number of ways of arranging 6 people in a circle = 5! = 120
Now, A cannot sit next to D and F simultaneously.
Let’s first find the number of arrangements in which A sits between D and F. In how many of these 120 ways will A be between D and F? Let’s consider that D, A and F form a single unit. We make DAF sit on any three consecutive seats in 1 way and make other 3 people sit in 3! ways (since the rest of the 3 seats are distinct). But D and F can swap places so the number of arrangements will actually be 2*3! = 12
In all, we can make A sit next to D and F simultaneously in 12 ways.
The number of arrangements in which A is not next to D and F simultaneously is 120 – 12 = 108.The second question is discussed as Question 3 in my post:
Question 3: There are 6 people, A, B, C, D, E and F. They have to sit around a circular table such that A can sit neither next to D nor next to F. How many such arrangements are possible?
Solution: In the previous question, A could sit next to D and F; the only problem was that A could not sit next to both of them at the same time. Here, A can sit next to neither D nor F. Generally, it is difficult to wrap your head around what someone cannot do. It is easier to consider what someone can do and go from there. A cannot sit next to D and F so he will sit next to two of B, C and E.
Let’s choose two out of B, C and E. In other words, let’s drop one of B, C and E. We can drop one of B, C and E in 3 ways (we can drop B or C or E). This means, we can choose two out of B, C and E in 3 ways (We will come back to choosing 2 people out of 3 when we work on combinations). Now, we can arrange the two selected people around A in 2 ways (say we choose B and C. We could have BAC or CAB). We make these three sit on any three consecutive seats in 1 way.
Number of ways of choosing two of B, C and E and arranging the chosen two with A = 3*2 = 6
The rest of the three people can sit in three distinct seats in 3! = 6 ways
Total number of ways in which A will sit next to only B, C or E (which means A will sit neither next to D nor next to F) = 6*6 = 36 ways
https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2011/1 ... ts-part-i/Let me know if this answers your question.