How can I solve this type of problems of permutation and combinations? Explain in detail?

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Solve Manually for some cases you will find this is none other than a fibonacci series :stuck_out_tongue:

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I asked such a Q in my time too, that how to tackle these type of problems.

The reply was that, generally these type of problems have some underlying trick/principle which we have to decipher. Because going brute force over these problems isnā€™t generally advised.

Also, should you required to calculate nCr or nPr values of big numbers, then theres a trick for that too. Usually such Q give ā€œprint result % 10^7+7ā€ or etc. Recall than (a x b)%n = [(a%n) x (b%n)]%n. Use the formula for calculating nCr or nPr by nCr-1 or such (Cant exactly recount) and store it modulo n. (Since in the end it wont matter whether you modulo it before multiplication or after multiplication. Eg- (5x4)%3=20%3=2, and (5%3 x 4%3)%3 = (2x1)%3= 2ā€¦ It doesnā€™t matter if I store 5 in the variable or 5%3. Similar holds here. So we can store those large nCr or nPr values should their use be required in formula.)

BTW- Lol, thanks naksh, even I was stuck on that Q. :stuck_out_tongue:

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this link would serve the purposeā€¦ there is a lot to learn here :slight_smile:

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