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Current time:0:00Total duration:3:46

CCSS Math: HSF.IF.B.4, HSF.IF.C.9

Charles and Tammy live the
same distance from work, and they started walking
to work at the same time. They both walked
at constant speeds, though not necessarily
at the same speed. Charles' distance from work is
shown in the following table. So at 50 seconds, he's
830 meters from work. Then another 50 seconds
goes by, and now he's 760 meters from work. Another 50 seconds
goes by, and he's 690 meters from work,
which makes sense. He's getting closer and closer
to work as seconds pass by. He's walking to work. Which of these sentences
could possibly be true? Select all that apply. So what are these
sentences talking about? So it looks like they're
all talking about Tammy. Tammy started 830
meters from work and walked towards work
at 2 meters per second. She started 900 meters from
work and walked towards work at 7/5 meters per second. So all of these seem
to be statements about where did Tammy start
and how quickly did she walk to work? So let's think about which of
these could possibly be true. Well, the one thing
that we know is that they live the same
distance from work. So the distance
that Charles started walking from work, that's
the same distance that Tammy started walking from work. So let's figure out
what that distance is. So we see that every
50 seconds, so if we go from 50 to 100 seconds, so
if we add 50 seconds, or 50 seconds go by, it
looks like Charles traveled another-- let's see. To go from 830 to 760,
his distance from work decreased by 70 meters. And let's see if that
keeps holding up. It should because they're
moving at constant speeds. Another 50 seconds goes by,
and his distance from work decreases by another 70 meters. So what would have happened
50 seconds in the past? So what would
happen at 0 seconds? So 50 seconds before
he's at 830 meters, he would have been 70
meters further from work. So he would have been
900 meters from work, and we see that it's consistent. When 50 seconds pass by,
when we go from 0 seconds to 50 seconds, he goes from 900
meters away to 70 meters away. So once again, his distance
from work decreases by 70. Then another 50 seconds goes
by, it decreases by 70 again. Another 50 seconds, it
decreases by 70 again. So at time 0, Charles was
900 meters away from work. Now, we know that Tammy and
Charles live the same distance away from work. So Tammy would have also started
900 meters away from work. Now, they tell us that
they don't necessarily walk at the same speed. So we don't know what
Tammy's speed is. This hasn't in any way
constrained Tammy's speed. So we know that she
started 900 meters away. So we know that this
can't be the case. That says 830. We know that this
can't be the case. We know that this can't be
the case because they all talk about starting at a
distance other than 900 meters. And over here,
both of these, they say she started at
900 meters from work and walked towards work
at 7/5 meters per second. Well, that's possible. It's not necessarily true. They just haven't given us
information on her speed. But given the
information so far, this is definitely
a possibility. Likewise, Tammy
started 900 meters from work-- we
know that is true-- and walked towards work
at 5/7 meters per second. Well, once again,
that's a possibility. That hasn't been ruled
out by the information right over here. So these two are
both possibly true.