How Should Bert Correctly Finish His Mental Subtraction Problem?

[mathdad]

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.

 

[MarkFL]

That's how my dad taught me to add/subtract when I was but a wee lad:

$$53-29=53-(30-1)=(53-30)+1=23+1=24$$

 

[HallsofIvy]

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.

 

[mathdad]

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.

 

[MarkFL]

You could also do it like this:

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

or:

$$53-29=53-(33-4)=(53-33)+4=20+4=24$$

 

[topsquark]

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