The natural number less than 1000 are, For three digits, The number of ways of filling hundredth place is 9 , The number of ways of filling tenth and once place are 9 and 8 respectively. Required number =9×9×8=648 For two digits, Tenth place can be filled with 9 , Required number =9×9=81 One digit, There are 9 one digit number. Total number of natural number less than 1000 is, 648+81+9=738