Mathematics - Number Theory - Modular arithmetic - Clock Arithmetic

Show a 12-hour clock. Ask: 'It's 10 o'clock. What time is it in 5 hours?' (3 o'clock, not 15!). 'What about 10+7?' (5). 'What about 10+26?' (12, then 2 more = 12). This IS modular arithmetic: 10+5 ≡ 3 (mod 12). Show a calculator where the student enters additions and sees results 'wrap around'. Then