Let's start with a simple one!

How many ways are there of distributing 10 marks among 3 questions so that every question must carry atleast 1 mark.


The answer is 36.

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3 solutions

Yash Singhal
Jun 3, 2014

After giving one mark to each question, 7 7 marks are to be distributed among 3 3 questions.So, the required number of ways are given by the number of non-negative integral solutions of x + y + z = 7 x+y+z=7 where x , y x,y and z z are the 3 3 questions.

Shouldn't it be "number of non-negative integral solutions of x + y + z = 7 x+y+z = 7 ", instead of positive?

Typically, I prefer the "positive solutions" approach, since it is easy to see that the answer is ( 9 2 ) {9 \choose 2 } .

Calvin Lin Staff - 6 years, 10 months ago

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Yes, a typo. Fixed now! Thanks for reminding.

Yash Singhal - 6 years, 6 months ago

Did the same!

Kartik Sharma - 6 years, 6 months ago
Monique .
Feb 6, 2015

If first question receives 1 mark, solutions are: 1,1,8 1,2,7 1,3,6 ... 1,8,1 --> 8 solutions If first question receives 2 marks, solutions are: 2,1,7 2,2,6 2,3,5 ... 2,7,1 --> 7 solutions ... If first receives 8 markts, solution is: 8,1,1 --> 1 solution

8+7+6+5+4+3+2+1 = 36

Melissa Quail
Jan 8, 2015

I just used stars and bars (see https://brilliant.org/wiki/integer-equations-star-and-bars/) where there are 2 bars and 9 gaps so there are 9C2 = 36 combinations.

how there are 9 gaps?

manu dude - 5 years, 11 months ago

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