Q.

Let A be a set of all 4 –digit natural numbers whose exactly one digit is 7. Then the probability that a randomly chosen element of A leaves remainder 2 when divided by 5 is

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a

15

b

122297

c

97297

d

29

answer is A.

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Detailed Solution

The number of four digit numbers having 7 as one digit in that four

 

If the number begins with 7, then the number of four-digit numbers is 1×9×9×9

 

The number of four-digit numbers, in which 7 is not in the first place and 7 must be one of the digits in the remaining three digits, is 8×3C1×9×9

Hence the total number four-digit numbers having 7 as one of the digits isn(s)=9×9×9+24×9×9=81×(33)=2673

Among all the above numbers, find the number of numbers whose unit digit is either 2 or 7

 

The number of four-digit numbers having 2 as unit digit and 7 as first digit is 81, the number of four digit numbers having 7 in unit digit is   8×9×9=648

And the number of four-digit numbers having 2 as unit digit and 7 at any place other than first digit place is 8×2C1×9=144

The number of four-digit numbers whose unit digit is either 2 or 7 isnA=81+648+144=873

 

Therefore, the required probability is nAnS=8732673=97297

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