🔍 Divisibility rules
A prime has exactly two factors: 1 and itself. Every other whole number above 1 is composite and can be built by multiplying primes together — that's its prime factorization.
2
Even last digit
Ends in 0, 2, 4, 6 or 8
Example: 348 ends in 8 → even → divisible by 2.
3
Sum of digits
Digit sum is a multiple of 3
Example: 471 → 4 + 7 + 1 = 12, a multiple of 3.
5
Ends in 0 or 5
Last digit is 0 or 5
Example: 235 ends in 5 → divisible by 5.
7
Double & subtract
Rest of the number minus 2 × last digit
Example: 203 → 20 − (2 × 3) = 14, a multiple of 7.
11
Alternating sum
Add and subtract digits in turn
Example: 209 → 2 − 0 + 9 = 11 → divisible by 11.
13
Try dividing
Divide by 13 and check the remainder
Example: Just try it: 13, 26, 39, 52, 65, 78, 91…