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Evaluating expressions with variables: temperature

In this example we have a formula for converting Celsius temperature to Fahrenheit. Let's substitute the variable with a value (Celsius temp) to get the degrees in Fahrenheit. Great problem to practice with us! Created by Sal Khan and Monterey Institute for Technology and Education.

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  • blobby green style avatar for user John Tims
    okay, when you placed a 1 under 25, why did you do that? is there a specific rule to this? or did it come from a part of the equation that i wasn't aware of?
    (21 votes)
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    • hopper cool style avatar for user Chuck Towle
      John,
      While it was not necessary, it was useful.
      Because any number divided by 1 is still that number, 25 = 25/1
      When working with fractions, any integer can be written in the form of the integer over 1
      You can then use the knowledge you have and rules you know regarding fractions to solve the equation.
      (34 votes)
  • aqualine ultimate style avatar for user teo
    How did he simplify 9/5 times 25/1?
    (15 votes)
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    • piceratops ultimate style avatar for user abassan
      H e used a shortcut and divided a factor in the numerator (25) and a factor in the denominator (5) by 5. That leaves 9/1 times 5/1 which equals 45
      If you don't use this shortcut, you get 225/5 which you can then simplify by dividing numerator and denominator by 5. That will give you 45/1 which equals 45.
      Hope this helps.
      (16 votes)
  • marcimus pink style avatar for user monayb1999
    okk now what happen to the 25 i got lost on that
    (13 votes)
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  • leaf green style avatar for user Perseus
    At the formula is given. How does this formula work?
    (3 votes)
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    • duskpin ultimate style avatar for user Patrick Stetz
      The formula is a conversion of Fahrenheit to Celsius. It's the same thing as the following equations...
      100 centimeters = 1 meter
      12 inches = 1 foot
      1 meter = 3.2808 feet
      1 British pound = 1.56 US dollars
      These formulas can find one of the variables as long as the other variable is known. Let's say I know that it is 90 degrees Fahrenheit out. I plug 90 in for 'F' and try and find 'C'
      F = (9/5)*C + 32 ----> 90 = (9/5)*C + 32 ----> 58 = (9/5)*C
      I subtracted 32 from both sides...
      58 = (9/5)*C ----> 32.2 = C
      The formula that I just used told me that 90 degrees Fahrenheit is the same as 32.2 degrees Celsius
      (16 votes)
  • starky ultimate style avatar for user KeenonTheDaywalker
    Okay please help me here! How does one multiply and divide with fractions? Please help!
    (3 votes)
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    • aqualine seedling style avatar for user Ethan
      To divide two fractions, I remember KFC. KFC represents KEEP, FLIP, CHANGE. Let's say we are dividing (2/3) / (4/5). We KEEP the first number the same. Then, we FLIP the sign from a division to a multiplication. Finally, we CHANGE the last number, or find the reciprocal of it. This gets us to (2/3) * (5/4).
      (11 votes)
  • spunky sam blue style avatar for user chaseshadow99
    What kind of fraction would you use the numbers 4, 6, 9, and 2?
    (3 votes)
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  • blobby green style avatar for user gmkogan
    I passed the practice. \how do I move to the next unit
    (3 votes)
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  • leaf green style avatar for user sg60847
    How did the formula f=9/5c+32 obtained ?
    (3 votes)
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  • blobby green style avatar for user Frank ussher
    need help don't understand the video.
    (4 votes)
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  • duskpin seedling style avatar for user Isra
    I Dont understand this one at all.
    Is that fraction always the same? Do we have to memorise it? Why those numbers? Why does he multiply the fraction by 25?, Isnt it a mixed number? Once he starts calculating the fraction and 25 I Have no idea what he is doing--
    Can someone Explain?
    (1 vote)
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    • piceratops sapling style avatar for user BNgetich69
      To answer your question
      *Is that fraction always same?*
      Yes, the fraction is always same because in the above equation it is a constant, though it can also be expressed as a mixed fraction i.e. 9/5 is also equal to 1.8 or 1 and 4/5. The above formula is very similar to that given for calculating the area of a a triangle.
      1 / 2 * b * h. In this case 1 / 2 is the constant (does not change) b and h are the variable (changes).

      *Do you have to memorize it?*
      No, the formula is usually given and you can either check it in books, or google it. Though it is a good idea to have it in your head.

      *Isn't it a mixed number?*
      Yes, it is a mixed number but expressed as an improper fraction.

      Therefore 9 / 5 C + 32 is really ((9 / 5 * C) + 32)
      Whereby 9 / 5 and 32 are constants and C is the variable.

      For example
      N.B. I am converting 9 / 5 to a decimal for ease of writing. 9 / 5 = 1.8

      F = 1.8C + 32

      When C = 25 degrees Celsius.
      F = 1.8 * 25 + 32 = 77

      When C = 35
      F = 1.8 * 35 + 32 = 95

      etc...
      (6 votes)

Video transcript

Express 25 degrees Celsius as a temperature in degrees Fahrenheit using the formula Fahrenheit, or F, is equal to 9/5 times the Celsius degrees plus 32. So they're telling us that we have 25 degrees Celsius. So we could put that in for C here, and we'll get the temperature in Fahrenheit degrees. So let's do that. So we'll get F is equal to 9/5-- now for C, we're going to put in 25-- times 25 plus 32. And now we can simplify this before we multiply 25 times 9. Remember, this is the same thing as 9/5 times 25/1. We can essentially divide the numerator and the denominator of our eventual product by 5. If we divide 25 by 5, we get 5. If we divide 5 by 5, we get 1. So this boils down to 9 times 5 plus 32. So our Fahrenheit degrees are going to be 9 times 5 is 45 plus 32 degrees. Or it's equal to 45 plus 32 is 77, so this is 77 degrees Fahrenheit.