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### Course: 6th grade>Unit 6

Lesson 7: Evaluating expressions word problems

# 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.

## Want to join the conversation?

• 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?
(27 votes)
• 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.
(46 votes)
• How did he simplify 9/5 times 25/1?
(19 votes)
• 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.
(23 votes)
• okk now what happen to the 25 i got lost on that
(15 votes)
• It got divided by 5, multiplied by 9, and 32 got added to it.
(5 votes)
• At the formula is given. How does this formula work?
(4 votes)
• 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)
• Okay please help me here! How does one multiply and divide with fractions? Please help!
(5 votes)
• 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).
(15 votes)
• What kind of fraction would you use the numbers 4, 6, 9, and 2?
(5 votes)
• how can you divide 9 by 90*60 plus 45 pie square.
(3 votes)
• I passed the practice. \how do I move to the next unit
(5 votes)
• Use the "next lesson" link at the bottom of the menu window on the left side of the screen.
(8 votes)
• What is the difference between F and C.I got confuse by that.
(3 votes)
• They represent different units of measure for temperature.
F = Degrees in Fahrenheit
C = Degrees in Celsius

Hope this helps
(11 votes)
• How did the formula f=9/5c+32 obtained ?
(4 votes)
• And 32 is the degrees Fahrenheit when it is 0 degrees Celcius.
(3 votes)
• Why do you divide 25 by 5 and not 9/5? Shouldn't you divide 5 on the same fraction?
(5 votes)
• You cancel out the fives. It is like simplifying the answer but doing it earlier.
(3 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.