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