63, because all others are perfect squares, or a whole number multiplied by itself is equal to that number 25 = 5 * 5 16 = 4 * 4 49 = 7 * 7 63 = (not a perfect square) 81 = 9 * 9
its 63 which is not like others ... you see 25 = 5*5 16 = 4*4 49 = 7*7 81 = 9*9 and .. 63 = 7*9 .... so from this we got that all the other numbers are perfect squares while 63 is not ..so it is not like the others ...
