How Should Bert Correctly Finish His Mental Subtraction Problem?

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Discussion Overview

The discussion centers around the mental subtraction problem of calculating 53 - 29. Participants explore various methods and strategies for performing this subtraction mentally, including different approaches to breaking down the numbers involved.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant questions Bert's initial approach of subtracting 30 instead of 29, suggesting that he needs to add back 1 to arrive at the correct answer of 24.
  • Another participant shares a method taught by their father, demonstrating that 53 - 29 can be calculated as 53 - (30 - 1), leading to the same result of 24.
  • A different participant reiterates that the answer is 24 and explains that adding 1 to 23 yields the correct difference.
  • One participant proposes alternative methods, including breaking down the subtraction into smaller components, such as 53 - (23 + 6) and 53 - (33 - 4), both resulting in 24.
  • A final participant introduces a Common Core approach, emphasizing the use of tens and breaking down the numbers into more manageable parts to arrive at 24.

Areas of Agreement / Disagreement

Participants generally agree that the correct answer to the subtraction problem is 24, but they present multiple methods to arrive at this conclusion. There is no consensus on a single best approach, as various strategies are discussed.

Contextual Notes

Some methods rely on specific breakdowns of the numbers, which may depend on individual teaching styles or mathematical frameworks, such as Common Core. The discussion does not resolve which method is superior or universally applicable.

mathdad
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To subtract 53 - 29 mentally, Bert subtracted 53 - 30 equals 23 first. What should Bert do next to find the difference?

This makes no sense.
 
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That's how my dad taught me to add/subtract when I was but a wee lad:

$$53-29=53-(30-1)=(53-30)+1=23+1=24$$
 
RTCNTC said:
To subtract 53 - 29 mentally, Bert subtracted 53 - 30 equals 23 first. What should Bert do next to find the difference?

This makes no sense.
Since he has subtracted one too many, 30 rather than 29, he needs to add one back: 23+ 1= 24 so 53- 29= 54.
 
My Solution:

When you subtract 29 from 53, the answer is 24.

What must be done to 23 to make 24?

If we add 1 to 23, the sum is 24, which is the difference between 53 and 29.

The next step is to add 1 to 23.
 
You could also do it like this:

$$53-29=53-(23+6)=(53-23)-6=30-6=24$$

or:

$$53-29=53-(33-4)=(53-33)+4=20+4=24$$
 
If we are using Common Core we would do it by tens:
53 - 29 = (50 + 3) - (30 - 1) = (50 - 30) + (3 + 1) = 20 + 4 = 24.

-Dan
 

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