Number detectives need a rule

Before pointing, ask for the rule

Four number cards are waiting for a detective. Which one does not belong? Before pointing, ask for the comparison rule. A number is not wrong just because it looks different. We need a reason that another person can check using the same information.

Try the even-number rule

Our first rule is even numbers. An even number can split into two equal whole-number groups with none left over. Test six, ten, thirteen and eighteen one by one. Do not choose the number with the longest name or the fanciest handwriting.

Explain the exception

Six, ten and eighteen divide into two equal whole-number groups. Thirteen leaves one over, so it breaks the even-number rule. Saying thirteen is not enough: explain the leftover. A reason turns a lucky guess into something another learner can inspect and understand.

Change only the rule

Keep the same four cards but change the rule to less than fifteen. Six, ten and thirteen now fit. Eighteen does not. No card changed its value. Only our question changed, so the exception changed too. That is not cheating; it is careful classification.

Pause: check a new list

Try a new list: four, eight, eleven and sixteen. The rule is multiples of four. Pause and choose the number that breaks it. You can count in steps of four or draw groups. Explain your choice before continuing to the answer.

Check with groups of four

Eleven is the exception. Four, eight and sixteen can be made from whole groups of four. Eleven falls between eight and twelve, so it does not fit. The odd-looking card was not our evidence; the group-size rule was. Always connect the answer back to that rule.

One list, two good questions

Fun fact: the same number list can have more than one defensible odd-one-out answer if the question uses different rules. In our first list, thirteen fails the even rule while eighteen fails the less-than-fifteen rule. The detective's most important clue is the rule itself.

Leave an evidence trail

Continue to the worksheet below the video. Write the rule, test each card and give a reason for the exception. Then try the second rule without changing the numbers. A good detective leaves an evidence trail, not just a mysterious circle around a favorite number.