No. of non negative solutions to x+y+z+w=20 is equal to no. of ways of arranging 20 identical balls and (4-1) separators. Therefore total no. of objects= 20 + 3 and no. of permutations= ( 20+3 )!/ (20)!. 3!
you have to run faster and faster so as to remain where you are.. even if you are on the right track you will get run over if you just keep sitting there...
Moderation Team
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