Medium
Which of the following numbers is different from the rest?
Explanation
Explanation
51 (5+1=6), 63 (6+3=9), and 15 (1+5=6) are all divisible by 3 because their digit sums are divisible by 3. For 23, 2+3=5, which is not divisible by 3. Therefore, 23 is different.
PRO TIP
Re-check your working by substituting the answer back into the original question.