Question

how many numbers lying beteeen 100 and 1000 can be formed with the digits 0,1,2,3,4,5 if the repetition of the digits is not allowed ?

Answer 1

umm....total number of 3-digit numbers... that can be formed...
so that is 5*5*4
=100

Answer 2

From 0,1,2,3,4,5 we have 4 choices for the first place = 1,2,3,4,5 

                                                                                           = 5 numbers.

Second place also has 5 choices except 0. So again 5 numbers.

Third place has 4 choices because 1 number is filled in 2nd place. so 4 numbers.

Total  numbers lying between 100 and 1000 = 5 * 5 * 4

                                                                           = 100

Hope this helps!