Single digit numbers are from 1 to 9 so in that total digits used = 9. From 10 to 99 there are 90 numbers of 2 digits , so here digit used = 90 
2 = 180. From 100 to 500 there are 401 numbers of 3 digits = 3 401 = 1203 (as all are of 3 digit number). So number of digits required to first 500 natural number = 9+180+1203 = 1392
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