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## 6th grade

### Course: 6th grade > Unit 6

Lesson 7: Evaluating expressions word problems- Evaluating expressions with variables: temperature
- Evaluating expressions with variables word problems
- Evaluating expressions with variables word problems
- Evaluating expressions with variables: cubes
- Evaluating expressions with variables: exponents

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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.

## 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?(23 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.(39 votes)

- How did he simplify 9/5 times 25/1?(18 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.(20 votes)

- okk now what happen to the 25 i got lost on that(16 votes)
- It got divided by 5, multiplied by 9, and 32 got added to it.(4 votes)

- At0:01the formula is given. How does this formula work?(5 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(18 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).(13 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.(2 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.(6 votes)

- How did the formula f=9/5c+32 obtained ?(4 votes)
- And 32 is the degrees Fahrenheit when it is 0 degrees Celcius.(2 votes)

- need help don't understand the video.(4 votes)
- Basically you are converting 25 Celsius to 77 Fahrenheit using the formula: F = 9/5 C + 32.(1 vote)

- 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)- 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.