i understood d logic but how can the total pairs be nc2 when it is given that the first no. will be less than the other one(i.e. i<j)…won't nc2 will include all the pairs of no., even when j is less than i…eg :(2,1)…
No, that would be nP2. When we do nC2, we are not ordering, just selecting, and in every selection there would be exactly one “increasing order” arrangement.
Thanks a ton for the article. 🙂 Apart from the concept , is there any shortcomings if we try to substitute the value of N here ? I tried to do it with n =4 and substituted the value of n in the options and got the correct answer . Wont substitution be a good option for these type of questions?
Yes KK it will be a valid approach 🙂 I went into the mathematical solution as I was focusing on the PnC-based approach! There are always many ways to solve any questions and options/substitution would definitely be one of your most powerful tools in a range of situations…
CAT-holics 
i understood d logic but how can the total pairs be nc2 when it is given that the first no. will be less than the other one(i.e. i<j)…won't nc2 will include all the pairs of no., even when j is less than i…eg :(2,1)…
No, that would be nP2. When we do nC2, we are not ordering, just selecting, and in every selection there would be exactly one “increasing order” arrangement.
regards
J
thnx…concept got clearer..
A classic example of using knowledge beyond the specific domain.
In the last question wouldn’t (3,2) also be a common friend ?
(2,3) has already been counted. (3,2) will not exist separately as i<j is a condition.
regards
J
Right! Thanks.
Hi J,
Thanks a ton for the article. 🙂 Apart from the concept , is there any shortcomings if we try to substitute the value of N here ? I tried to do it with n =4 and substituted the value of n in the options and got the correct answer . Wont substitution be a good option for these type of questions?
Yes KK it will be a valid approach 🙂 I went into the mathematical solution as I was focusing on the PnC-based approach! There are always many ways to solve any questions and options/substitution would definitely be one of your most powerful tools in a range of situations…
regards
J
Hi J,
Thanks a lot for all the articles.
Had a query,Won’t substituting the values of n be of a good option?