Using Mental Math Subtraction.

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Presentation transcript:

Using Mental Math Subtraction

$ 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? $ 152 - $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.

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. $ 152 - $17 = ? The new problem becomes $152 - $20. $ 152 - $20 = ?

$ 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? $ 152 - $17 = ? When we subtract $20 from $152 we get an answer of $132. $ 152 - $20 = $132 But wait, there’s more!

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? $ 152 - $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. $ 152 - $20 = $132 $ 132 + $3 = $135 The final cost of the jacket is $ 135.

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.

$ 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? $ 152 - $17 = ? Let’s round the $17 off to $20. $ 152 - $20 = ? Now, we will add a new step.

$ 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? $ 152 - $17 = ? Let’s round the $17 off to $20. $ 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.

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. $ 152 - $17 = ? + 3 + 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? $ 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.