Question

Number of natural numbers between 100 and 1000 such that at least one of their digits is 7, is?

Answer 1

107 , 117 , ... 197 10 natural numbers
170 171 ... 179 10 natural numbers , but we counted '177' two times ,
so there are 19 natural number at least one of their digits is 7 between 100-200
It's the same with 100 , 200 , 300 ,...900 , except 700-800
8*19=152
and there are 100 ones between700-800,
so answer is 252

Answer 2

including 100 and 1000 or not?

Answer 3

without 7: 8*9*9=648
total: 9*10*10=900

with 7: 900-648=252