13 thoughts on “PnC Examples – Selecting Seats 2

    • I assume you mean circular races; I can’t think of any significant value add I could provide in that 😦 Just the standard fare ou would find in any concept-book. If I do, rest assured I will post.


  1. Sir, what if in the first question..if 20 people were seated in a “circle” and we had to select 3 such that none of them are next to each other? Could you plz explain this scenario?

    • I’ll try to do a post on that situation at some point, though it is probably comfortably out of CAT scope. But basically, the thought process for selecting 3 seats out of 20 in a circle would be like: you select 3 seats, put 3 in between, and then put the other 14 in the 3 gaps in 16C2 ways. For people, slightly messier, I think one could make cases to be safe.


  2. Hi J,
    Please reply if there is anything wrong with my understanding. In the 1st ques, we are taking 5 regions as there can be 5 places(including the possibility of a chair before the first and after the last resp.) between the 4 chairs (i.e we are taking the 7 chairs as a single entity and dividing them in the rest ). Had it been given if one of the chairs was the last one or the first one , then we would have to partition in 4 regions as now there would be 4 possibilities (deducting one which would have come before first or last.)

    As always,thanks for the post. 🙂

    • Yes that is exactly right. Think of it as equivalent to distributing 16 chocolates to 5 kids such that only the corner ones can go away empty-handed. We would give the middle 3 kids 1 chocolate each and then distribute the remaining 13 among all 5 kids with no conditions whatsoever.


      • the second question from previous post which is solved here in a different approach of keeping at least 3 people in between them, so while of arranging 15 people, why we took 3 partitions only? shuldn’t it be 6? or we have considered those 3 as a single entity?

    • The 5 stops and the start and end points of the track (basically, the wording indicates that between the starting station and the first of the five stations also there has to be a gap)


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s