Of the numbers 18, 24, 26 and 36, three obey a rule that the remaining one breaks. Which is the odd one out?

Add the digits of each number and try that total as a divisor. For 18 the digits give 9 and 18 ÷ 9 = 2; for 24 they give 6 and 24 ÷ 6 = 4; for 36 they give 9 and 36 ÷ 9 = 4. For 26 the digits give 8, and 8 will not divide 26 evenly. A second reading picks out the same number, since 18, 24 and 36 are all multiples of 6 while 26 is not, so nothing else can be argued for.