Combinations 16213

From how many elements is it possible to create 120 second class combinations?

Correct answer:

n =  16

Step-by-step explanation:

C2(n)=k!(nk)!n!  n(n1)/2=120  n (n1)/2=120 0.5n20.5n120=0  a=0.5;b=0.5;c=120 D=b24ac=0.5240.5(120)=240.25 D>0  n1,2=2ab±D=10.5±240.25 n1,2=0.5±15.5 n1=16 n2=15   Factored form of the equation:  0.5(n16)(n+15)=0  n>0 n=n1=16

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