Subtracting Whole Numbers

Subtracting Whole Numbers

We're asked to subtract.
And we have the problem 68 - 42.

And what I want to do here, is
1) just show you how I would do this problem,

and then, talk a little bit
about why it actually works.

So if I have 68 - 42, one way to do it –
and this is probably the most typical way –

is to write the number you're
subtracting from on top,

and to write the number that you're
subtracting from that number below that.

So we're going to have 68 - 42.

And the thing you really want to think
about – or to make sure you do,

is to align your numbers according to place.

So the 2 is below the 8 –
(They're both in the 1's place.)

– and the 4 is below the 6.
(They're both in the 10's place.)

And later in this video it should be
clear – or I'll hopefully make it clear –

why that's a good thing to do.

Alright. So then we go to
the 1's place, and we see an 8.

And from that, we're
going to subtract a 2.

So, 8 - 2 is 6. (8 - 2 = 6)
I'll write that over here.

Let me write that over here.

8-2 is 6.

8 - 2 is equal to 6.
(Repeating: 8 - 2 = 6)

And then we go to the 10's place.
We have 6 - 4.

And since they're in the 10's place,
this is really 60 - 40.

But 6 - 4 is 2.
6 - 4 = 2.

And since these are in the 10's place,
this is really saying 60 - 40 = 20.

And I'll make that a little bit clearer in a second.

So we're actually done here.

68 - 42 = 26.

And you can check that for yourself.

If you add 26 + 42,
you should get 68.

Or if you subtract 26 from 68
you should get 42.

So I encourage you do that in
your own time after this video –

verify that 42 + 26 = 68.

And also verify what 68 - 26 is equal to.

And you should see that is equal to 42.

So these are two things for
you to check on your own.

Now, the last thing I want to do
in this video is just to explain

in a little bit more depth why this actually works.

And, at least in my mind, I like to visualize 68 –

(You won't to have write it out like this –

but it's one way to make sure you
really understand what is going on here.)

68 is the same thing as 60 + 8, or 60 and 8.

And from that, we're subtracting 42.

But 42 is the same thing as 40 + 2.

So we're subtracting 40,
and we're subtracting 2.

We're subtracting 42.
We're subtracting 40 and 2.

And so you can view this as
really two separate problems.

You have, in the 1's place, you have 8 - 2.

And that is exactly what we did over here.
You have a 6.

And then in 10's place,
you have 60 - 40, which is 20.

So you end up with 20 + 6.

and you know that that is
the same exact thing as 26.