Subtracting a mixed number from a whole number

- [Lecturer] Figure out what
seven minus 4 5/8 is equal to.

And pause this video
and try this on your own

before we do this together.

All right, now let's do this together.

Now there's different ways
that you could approach this,

but one way we could think

about lining up the whole numbers.

So we have seven,

and then from that we're
going to subtract four wholes

and 5/8.

Now we have an issue here

because this top number here
doesn't have any eighths.

So where do we subtract that 5/8 from?

Well, one thing that we could
do is we can rewrite seven.

Seven is the same thing as six plus one.

Why did I write it this way?

Because I can re-express one
as a certain number of eighths.

One is the same thing as what?

Well, one is the same thing as 8/8.

So seven is the same thing

as six plus eight over eight.

Why is this useful?

Well, I could write this,

and it's a little unusual
to write it this way,

but I could write this as 6 8/8.

I know that's a very
strange-looking mixed number,

but if we were to write it here,

it would actually make

the subtraction pretty straightforward.

If we rewrote the seven.

Let me change the color to
make it clear what I'm writing.

So if we were to rewrite
this seven as 9 8/8

and we from that,

we are subtracting 4 5/8.

The reason why this
whole exercise was useful

is now we could even think
of this as the eighth place

if you wanted to think of it that way.

But now we have enough eighths here

to subtract 5/8 from it.

8/8 minus minus 5/8.

This part right over here

is going to be equal to 3/8.

And then we could focus on the wholes.

So six wholes minus four wholes
is going to be two wholes,

and we're done.

2 3/8.

As I said,

there's other ways that you
could have expressed this.

You could have converted this second,

you could have converted both of them

into pure fractions
instead of mixed numbers.

But this is actually the way

that my head would probably handle it,

especially if I'm trying
to do it in everyday life.