Unit measurement word problem: weight (US customary) | Pre-Algebra | Khan Academy
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A standard elevator in a
mid-rise building can hold a maximum weight of
1 and 1/2 tons. Assuming an average adult weight
of 160 pounds, what is the maximum number of
adults who could safely ride the elevator? So what we need to do is we have
to get the maximum weight that the elevator could hold
in terms of pounds. And then say, OK, well, how
many adults is that? So they gave us the maximum
weight in terms of tons. So they say it is
1 and 1/2 tons. And it’s always easier to deal
with improper fractions than mixed numbers, so let’s write
this as an improper fraction. 1 and 1/2 tons is the same thing
as– well, 1 ton is 2 halves, and then you
add another 1 half, that’s 3 halves. So you get 3/2 tons. Or another way to think about
it is 2 times 1 is 2, plus 1 is 3. So the maximum capacity
is 3/2 tons. Let’s think about how
many pounds that is. And to do that, we have to
know that there are 2,000 pounds per ton. Let me write this up here. We know that there are
2,000 pounds per ton. This wasn’t given in the
problem anywhere. This is something I knew from
past experience, and it’s a good thing to know,
in general, that a ton is 2,000 pounds. So I’m going to write
it right over here. Now, how do we convert these
3/2 tons into pounds? Well, we’re going to multiply it
by something, and the units that we’re going to multiply
by, we want the tons to cancel out. So we’re going to want to have
tons in the denominator, so it cancels out with this
tons up here. And then we want pounds
in the numerator. And that’s exactly what
we wrote up here. There are 2,000 pounds for every
1 ton, or you could just say 2,000 pounds per ton. You could put the 1 there, but
it doesn’t really change the expression. Now, if we multiply 3/2 tons
times 2,000 pounds per ton, what’ll happen is that
the tons cancel out. That was the whole point of
multiplying it by this. And we would be left with 3/2
times 2,000, and the only unit left is pounds. And if you do it this way,
you’ll never get confused. You’ll know that the units
cancel out so you’re getting the right units. But if you just think about it
in your mind, it should also make sense. If there are 2,000 pounds per
ton and there are 1 and 1/2 tons, I should multiply 1 and
1/2 times 2,000 to get the number of pounds. That makes sense. And we know 1 and 1/2 times
2,000 is 3,000, but we’ll figure it out right here. So what is 3/2 times 2,000? Well, I just told you answer. We can actually simplify
it right over here. This is going to be 3 times
2,000/2 pounds. We can divide the numerator
and the denominator by 2. This will become 1,000, and this
will become 1, so it’s 3 times 1,000 pounds, or this
is equal to 3,000 pounds. So what we’ve done so far, we’ve
just figured out the maximum capacity of
the elevator. It can hold 1 and 1/2 tons,
which is the exact same thing as 3,000 pounds. Now, what we need to figure
out is 3,000 pounds is the equivalent to how many average
adults of 160 pounds? Or how many 160 pound people
would it take to weigh a total of 3,000 pounds? Well, we can just
divide by 160. And if you want to make sure
that the units work out, remember, we want our answer to
be in terms of people, and we want the pounds
to cancel out. So we have pounds in the
numerator here, so if we divide by pounds, the pounds
will cancel out. And then we want our leftover to
be people, or maybe person. Person, people, same thing. Let me do people. The grammar of it, singular,
plural, might make it a little confusing, but I think you
get the general idea. Now, if we were to write this
out, what does it tell us? 1 person weighs 160 pounds, so
there’s one person for every 160 pounds. So notice, if we multiply these
two expressions, the pounds will cancel out. We’ll be just left
with people. We’re multiplying 3,000 times
1/160, but it’s really just taking 3,000 and dividing by
160, which makes sense. We have a capacity of 3,000. Each of our people weighs
160 pounds. Divide by 160. It tells you how many people. But this way, you know that
the units are working out. So this is going to be equal
to 3,000/160 people. That is the maximum capacity
of the elevator in terms of average people. Now, what is this? Well, we can divide the
numerator and the denominator by 10. If we divide the numerator and
the denominator by 10, this becomes 300/16. If we divide 300 by 2,
this becomes 150. If we divide 16 by 2,
this becomes 8. Now, let’s see, what can
we do more here? We could divide by 2 again. Let me rewrite it. So this is the same thing
as 150/8 people. 150, we can divide by 2. It gives us 75. And if you take 8 divided
by 2, that is 4. So we have 75 divided
by 4 people. Let me just do that,
work it out. So you have 75 divided by 4. 4 goes into 7 one time. 1 times 4 is 4. You subtract. 7 minus 4 is 3. Bring down this 5. 4 goes into 35 eight times. 8 times 4 is 32. Subtract. 5 minutes 2 is 3. And then you have the decimal. We’re going to the right
of the ones place. We’re going to the
tenths place now. So we can bring down
a zero over here. 4 goes into 30 seven times. 7 times 4 is 28. You subtract. You get 2. Bring down another zero. 4 goes into 20 exactly
five times. 5 times 4 is 20, and
then we are done. So this expression is exactly–
the 3,000 divided by 160, or the 150 divided by 8,
the 75 divided by 4, it all turns out to be 18.75
people is the capacity of the elevator. 18.75 average 160-pound people
would weigh 3,000 pounds. Now, do they want a decimal? What is the maximum number of
adults who could safely ride the elevator? Well, if they’re all going to
be average, then the maximum number of adults, since you
can’t have 3/4 of a person, or 75/100 of a person,
the maximum number is going to be 18. If you have 19 average-weight
adults, you are going to be too heavy, and the elevator
will fall or something.

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