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i dont understand the step accompanied to the one i marked...i quite dont understand !

i dont understand the step accompanied to the one i marked...i quite dont understand !

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Grade:12

2 Answers

Vijay Mukati
askIITians Faculty 2590 Points
7 years ago
If you relate summation(r= 1 to 10) 10C10-r . 20Cr with the concepts from permutation and combination, Then we can say that this term represent, chossing 10 objects out of 30 in which 10 are of one type and 20 are of another type.
So directly we can write these as 30C10 but we are subtracting -1 because r starts from 1 not 0. Which means that all 10 can not be of one from the first 10 similar objects.

And same about the next term also.

Thanks.
Atul Saswat
30 Points
7 years ago
Thnx fr walking me through it..i gladly understood !
but i couldnt get this part : -->
Which means that all 10 can not be of one from the first 10 similar objects.
what does it mean ? 
thnx a lot...

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