The Art of Selection
Mastering Combinatorics
Welcome, future engineers! Today, we are going to tackle a classic combinatorics problem that often appears in JEE Advanced. It is not just about plugging numbers into a formula; it is about the art of logical partitioning.
Imagine you are the team selector for a prestigious cricket match. You have a pool of talent, strict rules to follow, and a team to build. Let us break this down step-by-step.
Phase 1
The Fixed Reality
The most common mistake students make is trying to select all ten players at once. But look closely at the problem: "One batsman and one bowler who are captain and vice-captain respectively of the team should be included." This is your anchor.
These two individuals are already on the team. They are not part of the "selection" process; they are the "given."
We started with 7 batsmen and 6 bowlers. Since we have fixed 1 batsman and 1 bowler, our remaining pool is 6 batsmen and 5 bowlers.
We needed a team of 10, but with 2 already fixed, we only need to select 8 more players. This is the first step to clarity: simplify the problem by handling the fixed elements first.
Phase 2
The Shrinking Pool and New Constraints
Now, let us look at the constraints. The original rule was at least 4 batsmen and at least 4 bowlers. Since we have already placed 1 batsman and 1 bowler in the team, our new requirement for the remaining 8 players is:
- We need at least 3 more batsmen (because 1+3=4).
- We need at least 3 more bowlers (because 1+3=4).
This is where the logic gets exciting. We have 8 slots to fill, and we must satisfy these new, tighter constraints. Let x be the number of additional batsmen and y be the number of additional bowlers. We know x+y=8, with x≥3 and y≥3.
Phase 3
The Case Study
To find the total number of ways, we must partition our problem into mutually exclusive cases. Let us list the possible values for (x,y) that satisfy our conditions:
Case 1: 3 Batsmen and 5 Bowlers
Here, x=3 and y=5. The number of ways to choose these is given by:
Case 2: 4 Batsmen and 4 Bowlers
Here, x=4 and y=4. The number of ways is given by:
Case 3: 5 Batsmen and 3 Bowlers
Here, x=5 and y=3. The number of ways is given by:
Why stop here? Could we have 6 batsmen and 2 bowlers? No, because that would violate the requirement of at least 3 additional bowlers.
Could we have 2 batsmen and 6 bowlers? No, that would violate the requirement of at least 3 additional batsmen. The boundaries are clear, and the logic is sound.
Phase 4
The Final Summation
Finally, we use the addition principle. Since these cases are mutually exclusive—you cannot have a team that is simultaneously (3B,5Bo) and (4B,4Bo)—we simply add the results:
And there you have it! The final answer is 155.
It is a beautiful example of how breaking a complex constraint into smaller, manageable cases leads you directly to the solution. Keep practicing this mindset, and you will find that even the most intimidating combinatorics problems become a thrilling puzzle to solve. You have got this!