Because even numbers are off, the rest are odd numbers, just look at multiples of 3 and 5, and then clear them, and the rest are on, because as long as they are multiples of 2, 4, 6, 8, multiples of 1, 3, 5, 7, and 9 below/kloc-0 digits are all mutually integrated, because multiples of 9 and 7 are also left. Ask for a multiple of 3, remember a rule: single digits and ten digits can divide 3 by its multiple! If the last number is 5 or 0, it is a multiple of 5!