Using Mental Math

Subtraction

2

This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?

$ 152 - $17 = ?

Let’s use the method for rounding off.

When using mental math, the goal is to makethe numbers easy to use.

In this problem let’s round the $ 17 off to $ 20.

3

This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?

$ 152 - $17 = ?

In this problem let’s round the $ 17 off to $ 20.

$ 152 - $20 = ?

The new problem becomes $152 - $20.

4

This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?

$ 152 - $17 = ?

$ 152 - $20 = $132

When we subtract $20 from $152 we get an answer of $132.

But wait, there’s more!

5

This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?

$ 152 - $17 = ?

$ 152 - $20 = $132

The problem tells us only to subtract $17. We subtracted $20. Therefore, we subtracted too much. Since $20 is $3 more than $17, we need to adjust our answer and add $3 back to our answer.

$ 132 + $3 = $135

The final cost of the jacket is $ 135.

6

This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?

Again, we will use rounding numbers to make an easier problem.

A Second Method for Subtacticon

7

This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?

Let’s round the $17 off to $20.

$ 152 - $17 = ?

$ 152 - $20 = ?

Now, we will add a new step.

8

This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?

Let’s round the $17 off to $20.

$ 152 - $17 = ?

$ 152 - $20 = ?

When we rounded the number 17 to 20, we needed to add 3 to it. Let’s add 3 to the 152 to keep the problem balanced.

9

This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?

$ 152 - $17 = ? + 3 + 3

When we rounded the number 17 to 20, we needed to add 3 to it. Let’s add 3 to the 152 to keep the problem balanced.

10

This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?

$ 152 - $17 = ? + 3 + 3$ 155 - $20 = ?

The new problem becomes $ 155 - $20. This is much easier to subtract in your head.

$ 155 - $20 = $ 135. Since we balanced the numbers by adding 3, we don’t need to adjust our answer at the end.