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- Amanda lives in Fairbanks, Alaska.
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The table below shows
the temperature changes
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in Fairbanks over a two-hour period.
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So when we went from 3pm to 4pm
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the temperature increased
by two degrees Celsius,
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and then from four to five the temperature
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decreased by five degrees Celsius.
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Amanda notices the temperature at 5pm
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is negative nine degrees Celsius.
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What was the temperature
in Fairbanks at 3pm?
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So they're essentially
telling us the temperature
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at the end of these changes,
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and they want us to
figure out the temperature
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at the beginning of these changes.
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So let's see if we can do that.
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Let me draw.
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Since, in thermometers we're
used to thinking vertically.
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So I'll draw a vertical number line here
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to think about this a little bit.
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So here we go.
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Alright, so at 5pm we end up at
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negative nine degrees Celsius,
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so let's just say this
is zero, negative one,
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two, three, four, five, six,
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seven, eight, nine.
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So we end up at negative
nine degrees Celsius.
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This is zero degrees
Celsius right over here.
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So from zero...
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Sorry, from 4pm to 5pm
the temperature dropped
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by five degrees Celsius
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and that got us to negative nine.
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So we drop by five to
get to negative nine,
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so we must've started
five above negative nine.
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So one, two, three, four, five...
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Right over here.
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So this is that drop.
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This is that five degree drop.
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Like we said, one, two, three, four, five.
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So that's that five degree
drop right over here.
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This is a negative five degrees Celsius.
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So at 4pm, right at 4pm, we
must've had a temperature of...
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Well, five above negative nine
is going to be negative four.
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Negative four degrees Celsius.
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This is what the temperature was at 4pm.
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So this is at 4pm.
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And then, to figure out 3pm,
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remember when we go from 3pm to 4pm
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our temperature increased
two degrees Celsius.
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So we increased by two
degrees Celsius from that hour
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to get to negative four degrees Celsius.
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So we increased by two degrees Celsius.
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So if we increase by two degrees Celsius
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to get to negative four, that means at 3pm
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we must've been two below that.
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So this is what we did from three to four.
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>From three to four we increased by two
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to get to negative four.
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So this right over here is
the plus two degrees Celsius
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right over there.
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So we must've started right here at 3pm.
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We must've started right there at 3pm.
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All I did is underline this.
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If you know we're at
negative nine to end with,
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and from four to five
we dropped negative five
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to get there, that means
at four we must've been
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five above negative nine.
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So we're at negative four,
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and then from three to four,
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if we increased two degrees to get there,
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that means at three we must've been
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two degrees below negative four.
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So this right over here is
negative six degrees Celsius.
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So what was the temperature
in Fairbanks at 3pm?
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It was negative six degrees Celsius.
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And you can verify that.
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If you start at negative six, you add two,
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you get to negative four,
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and then you subtract five,
you get to negative nine,
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which is exactly what we had here.