Show students how to model numbers using base 10 blocks. These blocks represent place value because there are three types of blocks: "units," which represent one, "rods," made up of 10 units, and "flats," made up of 100 units. Have them practice making two- and three-digit numbers using the base 10 blocks. For example, the number 73 is made up of seven rods and 3 units because it has seven 10s and three ones.
Model a subtraction problem with the base 10 blocks. Write the problem on the board and have students make the larger number with their blocks. For example, if the problem is 45 - 18, students will represent 45 with four rods and five units.
Explain to students that they will begin subtracting in the ones column of their problem. In this example, the ones column has a five and an eight. However, it is impossible to take eight away from five. Students will be able to see this is impossible by looking at their units in their base 10 representation. There are only five units there, so you cannot take away eight of them.
Show students how they can regroup, or "trade in," a rod for 10 units. Think of this as breaking a $10 bill. You would receive 10 $1 bills in exchange for your $10 bill, but you still have the same amount of money. When you regroup a number from the 10s place, you are adding 10 ones to the ones place. In our example of 45 - 18, students will regroup one of their four rods into 10 units and place those with the original five units. They are left with three rods and 15 units, still representing the number 45.
Rewrite the subtraction problem after regrouping. In this example, cross out the 4 in the number 45 and replace it with a 3 since you are left with three rods in the 10s place. Add a 1 before the 5, making it the number 15, since you added 10 units to the ones place.
Subtract the numbers in the ones column. Now that there are 15 units in this column, it is possible to subtract the eight ones that are in the number 18. You will be left with seven units in the ones column.
Subtract the numbers in the 10s column. You are left with three rods after regrouping, and you need to take away one because the number 18 has one 10. You will be left with two rods in the 10s place.
Ask students to look at how many rods and units are left to determine the answer to the subtraction problem. In this example, there are two 10s and seven ones remaining, so the answer to 45 - 18 is 27.