# Combinations

## 10 thoughts on “Combinations”

1. Sandeep |

“which will need to be replaced later” -made me chuckle

• That was rather the idea ðŸ™‚ Sometimes humour can make the concept easier to internalise…

regards
J

2. Sakshi Jain |

Shouldn’t the answer for the last question be 9*8*7*6?

• Only if you can distinguish one elephant from another, Sakshi ðŸ™‚ Can you?

The default convention is, people are considered distinct, monkeys or parrots or elephants are considered identical. Some simplifying assumptions must be allowed in maths, else every question would be an RC passage, a full-fledged legal document, to cover all the disclaimers!

regards
J

• Rohit Gupta |

But sir, seats would be distinct, right? 1 ele has 9 ways to sit in, the other one, though identical, won’t he have 8 ways since seats would be relative to the 1st ele? and so 9P4?

• Did I say the seats are identical? The elephants are.

regards
J

3. seema |

in second question, shouldn’t the prizes be distinct, because if they are identical then we dont need to do 3! and it would be 20P3, as in the 1st question, in which when the different roles were assigned, then we brought C (i.e.10C3) into picture.

• We are DIVIDING by 3! not multiplying. This number x 3! would be 20P3. Please read the previous posts carefully.

regards
J

4. Lakshman |

Hello Sir,

I felt that the question on a candidate attempting 6 rapid fire questions should be considered as a permutation question. The reason is that here, he may correctly answer any 6 among the available ten questions. Hence I felt even the arrangement should be considered.

He getting the first 6 questions right is different from he getting the last six right. Kindly let me know your thoughts.

• We are considering the 10 questions as distinct. If we had considered them identical, there would have been only 1 way of getting 6 right ðŸ™‚ Do note however, that we are choosing which 6 of the 10 distinct questions are being answered (and since they are being asked one after another, we cannot change the order in which the chosen 6 are answered even if we wished to).

Even when we are selecting 3 out of ten people in the earlier question, we didn’t say that the 10 people are identical; we said that we didn’t need to arrange the 3 selected people among themselves (or the 7 rejected people for that matter)

regards
J