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# the number of n digits number which consists of the digits 1&2 only if each digit is to be used atleast once  ,is equal to 510 then n is equal to(a)7(b)8(c)9 (d)10

Arun
25763 Points
3 years ago
Dear Ashish
$no. with atleast once occurrence \\=>total -no occurrence of 1 -no occurrence of 2 \\=>2^n-2=510 \\=>2^n=512 \\=>n=9$
Hope it helps, incase of any difficulty, feel free to ask
Thanks& Regards,