It is equivalent to finding how many such 2-digit numbers are there. In such numbers we cannot have 0 or 1 in unit’s place. When we have 2 in unit’s place, we have 1 such number, 12. When we have 3 in unit’s place, we have 2 such numbers, 13 and 23. . . . When we have 9 in unit’s place, we have 8 such numbers. So number of such numbers is (1 + 2 + 3 + ......+ 8)= 36. Hence the resulting number has 72 digits,