Divisibility & Prime Factorization

How the lesson starts: Divisibility & Prime Factorization

Gizmo's Robot Factory
Welcome to Gizmo's Robot Factory! Gizmo the robot builds every number out of special bricks called prime numbers. Today's order is the number 84. Which prime bricks is it made of? First, some quick divisibility tricks to speed up the job!
Divisibility tricks
Is 732 divisible by 3? Add the digits: 7 + 3 + 2 = 12, and 3 divides 12, so yes! By 4? Check the last two digits: 32 ÷ 4 = 8, yes! By 5? It must end in 0 or 5, so no. By 9? The digit sum 12 isn't a multiple of 9, so no.
Prime or composite?
A prime number has exactly two factors: 1 and itself. 17 and 29 are prime. A composite number has more factors. 21 is 3 × 7, and 35 is 5 × 7, so they're composite. And 1? It has only one factor, so it's neither!