Q. 7

If 15C3r = 15Cr + 3, find r.

Answer :

We know that if , then one of the following conditions need to be satisfied:


i. p = q


ii. n = p + q


Let us use condition (i),


3r = r + 3


2r = 3





We know that for a combination nCr,r≥0 and r should be an integer, which is not satisfies here,


So, the condition (ii) must be satisfied,


15 = 3r + r + 3


4r = 12



r = 4.


The value of r is 4.


Rate this question :

How useful is this solution?
We strive to provide quality solutions. Please rate us to serve you better.
Related Videos
Lecture on Combinations49 mins
Combinations - BOX & GAP Method38 mins
Check Your progress Part 2| Interactive Quiz: Permutation & CombinationFREE Class
Understand Permutations like never before60 mins
Fundamental Principle of Counting49 mins
Permutation & Combination (Lecture 4)FREE Class
Interactive Quiz on Combinations50 mins
Interactive Quiz on Combinations-02FREE Class
Circular permutationsFREE Class
Permutation & Combination (Lecture 6)FREE Class
Try our Mini CourseMaster Important Topics in 7 DaysLearn from IITians, NITians, Doctors & Academic Experts
Dedicated counsellor for each student
24X7 Doubt Resolution
Daily Report Card
Detailed Performance Evaluation
view all courses