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Current time:0:00Total duration:3:38

Video transcript

Amanda lives in Fairbanks Alaska the table below shows the temperature changes in Fairbanks over a two-hour period so when we went from 3 p.m. to 4 p.m. the temperature increased by 2 degrees Celsius and then from 4 to 5 the temperature decreased by 5 degrees Celsius Amanda notices the temperature at 5 p.m. is negative 9 degrees Celsius what was the temperature in Fairbanks at 3 p.m. 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 mmm alright so at 5 p.m. we end up at negative 9 degrees Celsius so let's just say this is 0 negative 1 2 3 3 4 5 6 7 8 9 so we end up at negative 9 degrees Celsius this is zero degrees Celsius right over here so from zero plus sorry from 4:00 p.m. to 5:00 p.m. the temperature dropped the temperature dropped by 5 degrees Celsius and that got us to negative 9 so we dropped by 5 to get to negative 9 so we must have started 5 above negative 9 so 1 2 3 4 5 right over here so this is that drop this is that 5 degree drawing said 1 2 3 4 5 so that's that 5 degree drop right over here this is an negative 5 degree Celsius so at 4:00 p.m. right at 4:00 p.m. we must have had a temperature of well 5 above negative 9 is going to be negative 4 negative 4 degrees Celsius this was what the temperature was at 4:00 p.m. so this is at 4:00 at 4:00 p.m. and then do you figure out 3:00 p.m. remember where we go from 3:00 p.m. to 4:00 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 from two by two degree Celsius to get to negative four that means at 3 p.m. we must have been to below that so this is what we did from 3 to 4 from 3 to 4 we increased by 2 to get to to get to negative 4 so this right over here is the plus 2 degrees Celsius right over there so we must have started right here at 3 p.m. we must have started right there at 3:00 p.m. all I did is unwind this if we know we're at negative 9 to end with and from 4 to 5 we dropped negative 5 to get there that means that 4 we must have been 5 above negative 9 so we were at negative 4 and then from 3 to 4 if we increased 2 degrees to get there that means at 3 we must have been 2 degrees below negative 4 so this right over here is negative 6 degrees Celsius so what was the temperature in Fairbanks at 3 p.m. it was negative 6 degrees Celsius and you can verify that if you start at negative 6 you add 2 you get to negative 4 and then you subtract 5 you get to negative 9 which is exactly what we had here