This is a free subtraction within 20 game for 1st and 2nd grade: a five-minute detective case you put on the board and solve as a class. It practices 1.OA.C.6 (subtract within 20) and 2.OA.B.2 (fluently subtract within 20). There are no logins, no student accounts and nothing to print unless you want the full case afterwards.
Why it works for subtraction within 20
Each clue is a subtraction the class has to work out before it can clear anyone: is the badge less than 15 − 6? Is 20 minus the badge an even number? Students subtract to find out which suspects go, so the practice is built into the play rather than bolted on.
How to run it
- Put it on the board. Tap Put it on the board above the case and it fills the screen. Six suspects, each wearing a badge number. One of them did it.
- Read clue 1 out loud. The class works out which badges the clue rules out. Tap each suspect it clears; a correct tap stamps them CLEARED, a wrong one just wobbles.
- Clues 2 and 3 unlock as you go. Keep clearing until one suspect is left, then tap them to close the case. The reveal explains every clue.
- Play it again. New lineup keeps the clues and shuffles the suspects. New clues gives you a fresh case. Round 2 and Round 3 are waiting in the tabs.
Three tips from the classroom
- Model the first clue on a number line or ten frame, then let the class do the rest.
- Ask students to say the difference before they tap, so the whole room hears the fact.
- Use the clock. A class that cleared a case in 1:40 on Monday wants to beat it on Thursday.
Making it easier or harder
For a class that finds subtraction hard, keep counters out and read each clue twice. For a class that is quick, give them a round each and time it.
Questions teachers ask
What numbers does it use?
Badges run from 1 to 18 and the clues use subtraction within 20, odd and even, and comparisons like less than 13 − 4.
Does it need student logins?
No. Open the page and play. There are no accounts, nothing to install and nothing to set up.
Is there a printable version?
The packs below are the printable version: a page of subtraction problems, a lineup of suspects, every right answer crosses one off. Answer keys included.
