Prime & Composite Numbers

How the lesson starts: Prime & Composite Numbers

Farmer Fern's garden
Hello, gardener! Farmer Fern plants flowers in neat rectangles. She has 12 sunflowers and 11 tulips. The sunflowers fit lots of rectangles. The tulips fit only one! Why? Let's dig in!
Prime or composite?
12 can be 1 × 12, 2 × 6 or 3 × 4. More than two factors: composite! 11 is only 1 × 11. Exactly two factors: prime! 17 is prime. 21 = 3 × 7, so it's composite. And 1 is neither!
Testing a number
Is 91 prime? Test the small primes. 2? No, 91 is odd. 3? The digits add to 10, so no. 5? It ends in 1, so no. 7? Yes! 7 × 13 = 91. So 91 is composite.