For a 3 digit number, 7 can be at the unit place, tens place or hundred place.
7 at hundred place, that means total combination will be:- 1*10*10 (first digit can only be seven but second and third can be any from 0-9)
7 at tens place, that means total combination will be:- 9*1*10 (first digit can be any number from 1-9, second will only be 7 and third will be any number from 0-9)
7 at unit place, that means total combination will be:- 9*10*1 (first digit can be any number from 1-9, second will
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7 at hundred place, that means total combination will be:- 1*10*10 (first digit can only be seven but second and third can be any from 0-9)
7 at tens place, that means total combination will be:- 9*1*10 (first digit can be any number from 1-9, second will only be 7 and third will be any number from 0-9)
7 at unit place, that means total combination will be:- 9*10*1 (first digit can be any number from 1-9, second will
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