Two-digit 5457

From how many digits can we create twenty-two-digit numbers in which the digits do not repeat?

Correct answer:

n =  5

Step-by-step explanation:

n (n1)=20  n (n1)=20 n2n20=0  a=1;b=1;c=20 D=b24ac=1241(20)=81 D>0  n1,2=2ab±D=21±81 n1,2=21±9 n1,2=0.5±4.5 n1=5 n2=4 n>0 n=n1=5

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