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Course: 8th grade (Eureka Math/EngageNY) > Unit 4

Lesson 4: Topic D: Systems of linear equations and their solutions

Converting Fahrenheit to Celsius

Converting Fahrenheit to Celsius. Created by Sal Khan and Monterey Institute for Technology and Education.

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  • piceratops ultimate style avatar for user Maino
    it is difficult the understand the formula of celsius and farenheit
    (15 votes)
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    • leaf green style avatar for user gary.devenish
      Just break the formula down in steps. Eg Farenheit to Celcius:

      Take Temp in Celcius, 1. divide by 5 (easy math), 2. then multiply by 9, 3. then add 32.
      So for 30 Celcius,
      step 1. 30/5 = 6.
      step 2. 6*9 = 54
      step 3. 54 + 32 = 86 F

      For the farenheit to Celcius, just reverse the order and do the opposite function in each steps:
      1. _subtract 32
      2. divide by 9
      3. multiply by 5
      Eg. So for 100 F =
      step 1 (subtract 32) 100 - 32 = 68
      step 2.(divide by 9) 68/9 = 7.56
      step 3. (mult by 5) 7.56*5 = 37.8 C

      Good rule of thumb is that Celcius is USUALLY a SMALLER number than Fahrenhet (until you are LESS than -40 F), that's a quick way to check to see if you did it right.
      (19 votes)
  • male robot donald style avatar for user Rick
    How do you calculate Farenheit to Kelvin?
    (4 votes)
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  • blobby green style avatar for user Tammy Campbell
    how is F and C equal at -40
    (4 votes)
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    • female robot grace style avatar for user merawaters
      Since they are both scales measuring the same thing and are somewhat proportional, they are bound to meet or cross at some point.

      If F = 9/5 C + 32,
      then at C = - 40, then 9/5 (-40) + 32 = -72 + 32 = -40 F

      Looking at the equation, we see that this point must meet 2 criteria
      - be below 0 ˚C
      Since the freezing point of water is higher than in the F scale than the C scale (32 ˚F vs. 0 ˚C) and both scales "grow" apart even further from there, the point of the scales crossing or intersecting would have to be below 0 ˚C, and below by more than 32˚ to bring the F scale down into negative territory as well

      - a multiple of "5" on the C scale
      So that the equation would give a whole number

      For this particular equation, there is only 1 number that satisfies these conditions and fits the equation and that is -40.
      (6 votes)
  • leaf blue style avatar for user David
    in the why is one? 10-9=9 not 1
    (3 votes)
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  • leaf green style avatar for user LazarKlincov
    Why are there still several units for basic things like distance, weight and temperature?
    Pros and cons?
    The only positive thing about using different systems is that you can adjust the scale for a certain purpose, let's take kelvin for instance. Temperature is a measurement of atom's movement, so basing a temperature scale on that specific property seems like a handy thing when working with chemistry.
    But in everyday life to me it seems like a better thing to share one standard system =)

    Any thoughts?
    (3 votes)
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    • female robot grace style avatar for user sbi
      I really agree with you on this. I feel like having a system that will make complete logical sense and be easy to remember/figure out would be so much better. Why is the US always the "wierd" one with things like this. I feel like the metric system and celsius would make things so much easier than having to do feet, and inches, and miles, and Farenheit.... just my opinion...
      (4 votes)
  • blobby green style avatar for user egwuchi
    why are temperatures greater for the fahrenheit scale than the celsius scale above 40 but lesser below 40?
    (2 votes)
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  • hopper happy style avatar for user Maryam
    Is there a video on thermocouples? Because I can't seem to find any.
    (2 votes)
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  • duskpin ultimate style avatar for user Emilia Mays
    Here is another great formula:

    Fahrenheit to Celsius: F - 32 X 5 divided by 9

    Celsius to Fahrenheit: C X by 9, then divide by 5, then add 32
    (2 votes)
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  • female robot ada style avatar for user sarahrwilcox
    I'm about to look super stupid, but I need help understanding Fahrenheit. I looked online and it states that Fahrenheit's freezing point is 39 degrees F. Then why is my fridge at 37 degrees F and nothing is frozen. Then my freezer is at 0 degrees F and everything is frozen. I don't understand. Why do we use Fahrenheit and not Celsius? HELP ME!
    (2 votes)
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  • blobby green style avatar for user DrMuhammad Idrees Khan
    one Fahrenheit equal to how much centigrade
    (2 votes)
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Video transcript

A thermometer in a science lab displays the temperature in both Celsius and Fahrenheit. If the mercury in the thermometer rises to 56 degrees Fahrenheit-- they're giving us the Fahrenheit temperature-- what is the corresponding Celsius temperature? And then they give us the two formulas, that if we know the Celsius temperature, how do we figure out the Fahrenheit temperature, or if we know the Fahrenheit temperature, how do we figure out the Celsius temperature. And these are actually derived from each other, and you'll learn more about that when you do algebra. And we also-- maybe in another video-- will explain how to derive these. It's actually kind of interesting, involves a little bit of algebra. But they gave us the formula. So they really just want us to apply it, and maybe make sure we understand which one we should apply. Well, they're giving us the Fahrenheit temperature right here, so F is going to be equal to 56. And they're asking us for the Celsius temperature, so we need to figure out what the Celsius temperature is. Well, in this one over here, if you know the Fahrenheit temperature, then you can solve for the Celsius temperature. So let's use this right over here. So our Celsius temperature is going to be 5/9 times the Fahrenheit temperature-- the Fahrenheit temperature is 56 degrees Fahrenheit-- minus 32. Well, 56 minus 32 is 24. So this is going to be equal to 5/9 times 24. And this is the same thing as 5 times 24, over 9. And before I even multiply out 5 times 24, we can divide the numerator and the denominator by 3. So let's do that. If we divide the numerator by 3 and the denominator, we're not changing the value. 24 divided by 3 is 8. 9 divided by 3 is 3. So it becomes 5 times 8, which is 40, over 3 degrees. And if we want to write this as a number that makes a little bit more sense in terms of temperature, let's divide 3 into 40 to get the number of degrees we have. 3 goes into 4 one time. 1 times 3 is 3. Subtract. 4 minus 3 is 1. Bring down the 0. 3 goes into 10 three times. 3 times 3 is 9. Subtract. Get a 1. And then you could bring down another 0. We now have a decimal point over here. You're going to get a 0 here. 3 goes into, once again, 10 three times. And this 3 is going to repeat forever. So you could view this-- this is equal to 13.333-- it'll just keep repeating. This little line on top means repeating-- degrees Celsius. Or you could say that, look, 3 goes into 40 13 times with a remainder of 1. So you could say that this is also equal to 13, remainder 1. So 13 and 1/3 degrees Celsius. Either way, it works. But that's our Celsius temperature when our Fahrenheit temperature is 56 degrees.