Question

How many numbers are there between 100 and 1000 such that 7 is in the units place? (repetition of digits is allowed)

Answer 1

Here there three places which are to be filled with numbers upto { 1.............9 ,0}
And the unit place is already occupied by 7
So the digit at tens place can be filled in 10 ways .
And the digit At hundred place can be filled in 9 ways , as 0 can not be there .

Hence the numbers are 10 *9 = 90 numbers which have 7 at unit place .