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In this lesson, we will continue with the two-digit arithmetic we were studying in the last lesson, now looking at subtraction and multiplication.
How many green squares are there on each of these grids? As in the last lesson, use the Rearrange button to make them easier to count. For example: in the first row of the table, the Rearrange button splits the 79 into 70 + 9, and groups the 4 from the 34 with the 9.
How many green squares are there on each grid? Again, use the Rearrange button if it is helpful. For example: in the first row of the table, the Rearrange button splits the 86 into 70 + 16, and groups the 8 from the 18 with the 16 from the 86.
Solve these subtraction problems in the standard way, using columns instead of the grid.
As in the addition lesson, notice that the normal “column” method of subtracting two numbers involves exactly the same steps as the grid illustrates! That is, the grid allows you to see why the normal method works.
Each of these multiplication problems is illustrated on the grid. Count the number of small green squares in each example. Use the Rearrange button to switch between the original picture and an equivalent picture where the squares are easier to count.
Solve these multiplication problems in the standard way, using columns instead of the grid.
As with the addition and subtraction methods, notice that the Rearrange button illustrates on the grid the steps you perform when multiplying.