This is a free addition 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 (add within 20) and 2.OA.B.2 (fluently add within 20). There are no logins, no student accounts and nothing to print unless you want the full case afterwards.
Why it works for addition within 20
The clues are short sums the class works out together: is the badge more than 7 + 6? Is it a double? Is it 10 or more? Each clue clears some suspects and the whole room is adding to find out which. It is five minutes of fact practice that looks like a game.
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
- Hold up fingers, use the ten frame on the wall, or count on. Any strategy that gets to the sum is fine; the game only cares about the answer.
- Ask which badges does that clear? and let several students tap. The badge wobbles if it is wrong and nothing is lost.
- Say the badge numbers out loud before you start. Hearing the six numbers helps younger students hold them in their heads.
Making it easier or harder
For a class still working within 10, read each clue and let them use counters before they tap. For a class ready for more, the place value game uses three-digit numbers.
Questions teachers ask
What numbers does it use?
Badges are 5 to 20 and the clues use sums within 20, doubles, tens, odd and even, and the last digit. Every lineup is different.
Is it good for a morning meeting?
That is where it fits best: a two-minute round while everyone settles, with the class calling out the answers.
Is there a printable version?
The packs below are the printable version: a page of addition problems, a lineup of suspects, every right answer crosses one off. Answer keys included.
