Archive

• 2026-06-21Handshakes and triangular numbers
• 2026-03-08Estimating square roots in your head
• 2026-01-17Monty Hall, counted by hand
• 2025-11-22The birthday problem without formulas
• 2025-09-06A divisibility test for 7 that is actually usable
• 2025-07-19Casting out nines
• 2025-05-30Squaring numbers that end in 5
• 2025-04-12Multiplying by 11 in your head