Each set consists of numbers which are obtained by multiplying a certain number by 9, 7 and 5 respectively.
Thus, in the given set, 63 = 7 * 9, 49 = 7 * 7, 35 = 7 * 5.
Similarly, 81 = 9 * 9, 63 = 9 * 7, 45 = 9 * 5.