What are the prime and composite numbers from 1 to 100

A prime number has exactly two factors, 1 and itself. A composite number has more. The trouble is that most kids test only 2, 3 and 5, and an odd number that survives those three feels prime.

That is why the sheet mixes in odd composite numbers such as 49, 77 and 91. Each one hides a factor of 7: 49 is 7 x 7, 77 is 7 x 11, 91 is 7 x 13. The second set repeats several numbers from the first, such as 7, 13, 19, 21 and 51, so your child meets the same number with the opposite question.

This practice rests on 4.OA.B.4, the Grade 4 standard for factors, primes and composites up to 100. Sixth graders need it again before greatest common factor and least common multiple work.

Give your child one test to run on every tag: "Can it split into equal rows of 2 or more?" If it can, it is composite. Start with 51. It is odd and ends in 1, so it looks prime, but 5 + 1 = 6, and that means 3 divides it: 51 is 3 x 17.

When they reach 2, ask, "What are the factors of 2?" Some kids skip it because even numbers feel composite. Let them say why 2 belongs with the primes.

A note for you: the solution page misses three answers. 23 is prime in the first set, and 69 and 93 are composite in the second, since 69 is 3 x 23 and 93 is 3 x 31. Hunt for those three together.

Primes turn up whenever you arrange things in neat rows. Next time you bake, hand your child 13 cookies and a tray: "Can you make a rectangle with more than one row?" They cannot, and that is what prime means. Try 12 and watch the options pile up.

At dinner, count the forks in the drawer and ask whether they could be set out in equal rows.

For reading together, Spaghetti and Meatballs for All! by Marilyn Burns follows a family reunion where 32 guests keep pushing square tables around. It is a story about area, and it also shows how many ways one number can be arranged.

Close by naming the work: "You tested every odd number instead of guessing." Next in this series, the prime factorization sheet breaks composites into their primes.