378
One digit numbers from 1 to 9 = 9
Two digit numbers from 10 to 99 = 9 × 9 = 81
(tens digit can take values 1 to 9, units digit can
take remaining 8 values as well as 0)
Three digit numbers from 100 to 500 = 4 × 9 × 8
= 288
(hundreds digit can take values 1 to 4, tens digit
can take 9 values and units digit can take any one
of the remaining 8 values)
All positive integers up to 500 with non repeating
digits = 9 + 81 + 288 = 378.
2

1

1

2

1

2

4

1

8

2

70

1

3

90

30

3

3

2

4

9

2
