approach

For every digit to appear from 1 to 99 -> 20 times any digit will appear

similarly for 100,200,300,400,500,600,800,900 -> total 8*20=160 (Because hundred will remain constant and we know the fact that for 1to99 20 times a digit will appear)

For 700: There will be 100 digits for 7-> 100

For7XX there will be 20 ways ->20

Total (20+160+100+20) there will be 300 digits

I hope it helps a bit.

Press Kudos :P