Combinatorics? PLEASE help!

Question

Guys,
I am stuck on some questions in Combinatorics: when I have to multiply, add, divide substruct combinations?
Well, I can calculate for example 4 combinations out of 8, but when I see explanations when combinations are add or multiplied I am really confused.

Could you please share links or post here some study materials.

I am really in depression now.

ezhilkumarank · Answer

Buddy, you are not alone. So cheer up ... :-D

.

PTK · Answer

Glad to know :wink:

Here is a Combinatorics thread but it is still unfinished. math-combinatorics-87345.html Let's wait for gurus.

frank1 · Answer

i also have problem in Combinatorics+probablity problems of 700-800 level...either it gets too long or i dont reach to correct answer....
i think most of the time...exceptions and "particular approach to be applied for particular situation" creates problems...

jpr200012 · Answer

These were hard for me at first, but they got a lot easier they second time I read and the section in the MGMAT Word Translations book. They explained the concepts through use of examples by using anagrams. It seemed overly complicated at first, but finally it sunk in. If you access to this book, it is very helpful. In another chapter, they give the formulas for combinations and permutations, but I haven't relied on using those. I haven't found any material online comparable to the stuff in that book.

jpr200012 · Answer

Some things that helped me were:

1. If there is a constraint on one of the choices, do it first even if you wouldn't actually make that choice first in the actual activity. For instance, if you have to pick 3 unique numbers, and the last one must be odd, then start with the final choice of the odd number.

2. An example of when you must multiply and subtract: How many ways can you arrange objects a,b,c,d, and e if c and d can't be next to each other. First ignore the constraint and just figure out the number of ways they can be arranged. Now consider c and d as "one" thing and how many ways can you arrange a, b, cd, and e? This is how many possibilities that c and d are arranged next to each other with c on the left, and d on the right. Now multiply this number by 2, because you have to consider d on the left and c on the right. Now subtract this from the original number you got at the beginning.

3. An example when you divide: If you pick from a group of 11 and choose 4, how many groups of 4 can be chosen when order doesn't matter? Start with 11! in the numerator. Since order doesn't matter, the group of 4 is not distinguishable and neither is the group of 7 that you didn't pick. Therefore, you must divide out 4! and 7!.

The more problems you work, you can start to recognize these patterns.

sandeep800 · Answer

i will try to find some doc of that type Pkit soon

PTK · Answer

guys, please refer this link: math-combinatorics-87345.html#p785213 One of the mebers contributed to our knowledge:)

Combinatorics? PLEASE help!

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