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## 4th grade

# Finding reasonable unit of measurement example

CCSS.Math:

Sal chooses reasonable units of time activities like baking cookies. Created by Sal Khan.

## Video transcript

Identify the most
reasonable units to measure each of
the following choices. So the time it takes to
bake one pan of cookies. Let's see, that's
usually in the minutes, so I'll go with
minutes unless you're doing some type of slow baking. But I'll say minutes is
a reasonable unit here. The time it takes to
pour a cup of coffee. Well, that might take you,
at most, 1 or 2 seconds. So you definitely don't want
to do it in hours or years. And we already used up
minutes, and you definitely don't want to use
minutes either. It's only a few seconds. The time it takes to drive
from Chicago to Saint Louis. Well, that hopefully
won't take you years. That'll take you-- I don't
know the exact distance, but it's going to take you
several hours to do that. So I will go with hours. Let's do a few more of these. Identify the most
reasonable units to measure each of the
following choices below. So we have kilometers,
meters, and millimeters. And a kilometer, of
course, kilo tells us that's equivalent
to 1,000 meters. And milli means
one-thousandth of a meter. Or another way of
thinking about it, a meter is 1,000 millimeters
and a kilometer is 1,000 meters. The length of a ladybug. Well, the length, unless it's
some kind of monster ladybug, we're probably thinking
in millimeters. The height of an average person. So an average person
is multiple meters. It's going to be at least 1
meter tall and more likely closer to 2 meters. Especially your average
male, adult male, would be right
around 2 meters tall. So I would go with meters. The length of a marathon. So this is a long-distance
race we're talking about here. If you're used to thinking
in US customary units, a marathon is 26 miles. A kilometer isn't
quite equal to a mile. 1.6 kilometers is
equal to a mile, but it also makes sense to
measure this in kilometers. You would measure the
length of a marathon in miles or kilometers. You'd measure the height
of an average person, if you're using US customary
units, you would do it in feet. If you're using metric units,
you would do it in meters. The distance between San
Francisco and Los Angeles. So once again, this is
several thousand kilometers. We'll go with kilometers. Or actually, San
Francisco to Los Angeles is several hundred kilometers. I thought that said New
York to Los Angeles. San Francisco to Los Angeles,
several hundred kilometers, but still you want
to do kilometers. The length of an
average-sized swimming pool. Well, meters seems like a
reasonable measure there. Your average pool might
be 25 meters or 50 meters or something like that. The thickness of
a slice of bread. So a slice of bread, it's
definitely a lot thinner than a meter, so I'm going
to go with the millimeters. Let me check our answer. That right. Let's do one more. Identify the most
reasonable units to measure the weight of each
of the following choices below. So see, we have
ounces and pounds. And just as a reminder, 1
pound is equal to 16 ounces or an ounce is equal to
a sixteenth of a pound. So a couch-- well, that's going
to be many, many, many pounds, probably over 100 pounds. So we definitely want to put
this in the pounds category. A cookie-- unless it's
a really large cookie, it's going to weigh
less than a pound. So I'm comfortable
going-- it seems reasonable to go with ounces. A television-- well, they're
getting lighter every day, but they still weigh a good bit. They still weigh
multiple pounds, usually multiple tens of pounds. So we'll go with pounds. A cracker-- and once
again, unless we're talking about a
really huge cracker, most crackers are
very, very light, lighter than even a cookie,
so we will go with ounces.