GIVEN: Numbers from 0 to 1000. CALCULATION: Since from 0 to 1000 only two – digits and three digits numbers appeared. Digits ‘1’ and ‘2’ can appear either at first two places or at last two places. Between 0 – 100: only 12 is the number in which digits ‘1’ and ‘2’ appears together. Between 101–200:112,120,121,122,123,124,125,126,127,128 and 129 = 11 Between 201–1000:212,312,412,512,612,712,812 and 912 = 8 Required number = 1 + 11 + 8 = 20