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Course: Algebra (all content)>Unit 1

Lesson 4: 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.

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

Hope this helps