Magic tricks to know if it divides.
Shortcuts to tell if a number is divisible by 2, 3, 5, etc.
Discovered by number theorists to speed up calculations.
Even? Divides by 2. Ends in 0 or 5? Divides by 5. Digits add to 3? Divides by 3.
Plain English: You can look at a huge number like 1,000,005 and instantly know it divides by 5 because it ends in 5. These are secret shortcuts.
Real-world example: Checking if you can split a bill evenly without needing pennies.
Confusing the '3 rule' (sum of digits) with the '9 rule' (also sum of digits).