Zero as a placeholder

Worked solution

1. 1

   Record the value of the hundreds.

   $5\text{hundreds}=500$

   Five hundreds must occupy the hundreds place.

2. 2

   Include the empty tens place and the ones.

   $500+0+9=509$

   A zero between $5$ and $9$ keeps the $9$ in the ones place.

Worked solution

1. 1

   Split the first number.

   $320=300+20$

   In $320$, the $2$ counts tens and the zero shows no ones.

2. 2

   Split the second number.

   $302=300+2$

   In $302$, the zero shows no tens and the $2$ counts ones.

3. 3

   Compare the parts that differ.

   $20-2=18$

   Both numbers have $300$, so the difference comes from twenty compared with two.

4. 4

   State how the numbers differ.

   $320=302+18$

   Moving the $2$ from ones to tens makes the number eighteen greater.