- [Instructor] Today
we're gonna talk about
how to round to the nearest ten
using vertical number lines.
So a vertical number line
is just like another regular number line
except it goes up and down, or vertical,
instead of horizontal.
Vertical number lines are
really helpful in rounding
because it lets us compare numbers
in a way that we're used
to, going up and down,
the lower numbers are on the bottom,
and the higher numbers are on the top.
So let's say that I have the number 53
and I need to round
this to the nearest ten.
Sometimes you'll see the symbol
for rounding look like this,
a squiggly arrow.
Using a vertical number line,
I put the ten below 53 on the bottom,
so the ten below 53 is 50.
The ten above 53 goes on the top line,
that would be 60.
So it's just like counting by tens,
where would 53 fall in the middle?
The middle line becomes
55, the exact midpoint.
Now I need to place 53.
I'm gonna draw my ones on here,
51, 52, 53, 54,
and I can mark 53 right there.
Now I can see that 53 falls below 55,
so when I'm rounding, I
need to round it to 50.
If it had been above 55,
I would've round up to 60.
If it had landed exactly on
55, if I was rounding 55,
we round up.
There's no reason for this except the fact
that we all agree that
this is how we do it,
because it would be very confusing
if people got to choose if
they round 55 up or down,
because it's exactly in the middle,
it's not closer to 50 or closer to 60.
So we have a rule that says
if it ends in a five, we round up.
I'm gonna make a note of that right here.
Round up.
I can even add more
notes to our number line.
Say from here to here round up.
And from here to here round down.
So the steps were
number one,
the bottom tick mark
we'll say mark,
is the ten
below.
Number two,
the top mark
oops, that shouldn't be then,
is the ten above.
Okay that's it for today.
Thanks for watching.
