# CAT questions: Permutations & Combinations - Videos

9
144

https://www.dropbox.com/s/mw6bgkr5mo1sgy1/CAT%20questions-%20P%26C.pdf?dl=0

source

1. Greetings sir,
I've tried to come to a formula for questions like Q4.
The sum of all the number of a N digit number will be
(Sum of digits)x(N-1)!x(111..n times)
i might have read it somewhere but it just struck me right now while solving, let me know if there is any alterations required in this.
Though this works only when the digits are non repeating and there is no 0 in the digits given.

2. for question 10 we can do:- 21c3 – 10c3 – 11c3

21c3 :- Meaning forming triangles from 21 points of which no 3 points are co-linear.

(10c3) : Meaning subtracting those triangle which could have been formed from 10 points of which no 3 are co-linear but here these triangles could not be formed as all 10 points are co-linear

(11c3): Meaning subtracting those triangle which could have been formed from 11 points of which no 3 are co-linear but here these triangles could not be formed as all 11 points are co-linear