Units digit can have 2 possible values (0,1) and the 100th digit can have 1 possible value (1) therefore, numbers possible is 1X2
When, middle number's value is 3. Units digit can have 3 possible values (0,1,2) and the 100th digit can have 2 possible values (1,2) therefore, numbers possible is 2X3
And so on....(units digit value could be 2....9)
Hence, total no. of possible numbers are: 1X2 + 2X3 + .........+ 7X8 + 8X9 = 240