Number Riddles — Factors, Multiples & Primes (Grades 5–6)
Number riddles combine clues — "I'm a prime number between 10 and 20, and my digits add to 4" — and you use everything you know about factors, multiples, and primes to narrow down the one number that fits every clue. The strategy: list the numbers that satisfy the first clue, then cross off any that fail the next, until one remains.
Understanding number riddles — factors, multiples & primes
Number riddles combine clues — "I'm a prime number between 10 and 20, and my digits add to 4" — and you use everything you know about factors, multiples, and primes to narrow down the one number that fits every clue. The strategy: list the numbers that satisfy the first clue, then cross off any that fail the next, until one remains.
Key Idea
Number riddles combine clues — "I'm a prime number between 10 and 20, and my digits add to 4" — and you use everything you know about factors, multiples, and primes to narrow down the one number that fits every clue. The strategy: list the numbers that satisfy the first clue, then cross off any that fail the next, until one remains.
Seeing it in action
Worked example
"I'm a prime between 10 and 20, and my digits add to 4." Who am I?
Primes between 10 and 20: 11, 13, 17, 19.
Digit sums: 11→2, 13→4 ✓, 17→8, 19→10. → 13.
Cross off clues until one number remains.
Try a few
"Even number, factor of 12, greater than 5."
both fit; if "less than 10"
"Multiple of 5, less than 20, digits add to 5."
5; 50 too big.
"Prime, one-digit, odd, more than 5."
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