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