SBI Clerk Mains Written Test Qs.(Quantitative Aptitude) :: Problems on Numbers - Discussion

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Explanation:
It's a permutation question.
there are totally 10 numbers and in which 3 and 5 are already fixed as first two digits.
hence total numbers n=8
and the remaining digits which are needed to be formed its 4 digits apart from the first two digits already formed in the 6 digit phone number.
so r=4
permutation formula npr=n!/(n-r)!
=8!/4!
=8*7*6*5*4*3*2*1/4*3*2*1
=8*7*6*5
=1680