1. Using Mental Math
Subtraction

2. $ 152 - $17 = ? Let’s use the method for rounding off.
This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?
$ $17 = ?
Let’s use the method for rounding off.
When using mental math, the goal is to make
the 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
This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?
In this problem let’s round the $ 17 off to $ 20.
$ $17 = ?
The new problem becomes $152 - $20.
$ $20 = ?

4. $ 152 - $17 = ? $ 152 - $20 = $132 But wait, there’s more!
This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?
$ $17 = ?
When we subtract $20 from $152 we get an answer of $132.
$ $20 = $132
But wait, there’s more!

5. The final cost of the jacket is $ 135.
This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?
$ $17 = ?
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.
$ $20 = $132
$ $3 = $135
The final cost of the jacket is $ 135.

6. A Second Method for Subtacticon
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.

7. $ 152 - $20 = ? $ 152 - $17 = ? Let’s round the $17 off to $20.
This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?
$ $17 = ?
Let’s round the $17 off to $20.
$ $20 = ?
Now, we will add a new step.

8. $ 152 - $20 = ? $ 152 - $17 = ? Let’s round the $17 off to $20.
This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?
$ $17 = ?
Let’s round the $17 off to $20.
$ $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
This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?
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.
$ $17 = ?

10. This jacket costs $152. It is on sale for $17 off
This jacket costs $152. It is on sale for $17 off. Using mental math, how much will this jacket cost?
$ $17 = ?
$ $20 = ?
The new problem becomes $ $20. This is much easier to subtract in your head.
$ $20 = $ 135. Since we balanced the numbers by adding 3, we don’t need to adjust our answer at the end.