Arithmagons puts numbers on the corners of a triangle and sums on the edges. Players decide which number belongs in each circle so every edge equals the sum of the two corners it connects.
Early puzzles are addition and number bonds. Later ones ask students to work backwards from a known sum, try a value, and revise, which is the same inverse thinking used in missing-addend problems. Some boards grow into larger figures with more shared corners, so one choice affects several edges at once.
PlayMath.org and the Julia Robinson Mathematics Festival have partnered to bring some of the real-world activities from the math festivals to life online. JRMF is a nonprofit that hosts non-competitive math festivals, where K–12 students explore hands-on puzzles together.