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# Evaluating an expression with one variable

Learn how to evaluate an expression with variables using a technique called substitution (or "plugging in"). Created by Sal Khan and Monterey Institute for Technology and Education.

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• Change 90% into a decimal: 0.90 or 0.9
Translate the key words: "of" = multiply and "is" means "="
Translate the phrase: "8 is 90% of n" becomes 8 = 0.90n
Solve for "n" by dividing both sides by 0.9

Alternatively, set up a proportion:
8/n = 90/100
Cross multiply and divide to solve for "n"

Hope this helps.
• If t=11, what is the answer?
• At , is 5. (1) not the same as 5 x x 1 ?
• If another operation is before the parentheses, then you don't multiply. If you have something like 3- (-3), you just subtract the -3.
If yo have 4*(5), you only multiply once.
• Why is X the most used variable is it because of the x and y axis or some other reason
• yes, it is! x and y are commonly used as variables on khan, but what they are most famous for is the the x and y axis, but as sal says "You can use any variable but were going to use x and y" they just use it on khan for equations and other math problems often! btw off topic, im a fellow programmer two! nice to meet you!:D
• I have 2 questions:
1. Are variables called variables because they can vary from time to time?
2. ANYONE TELL ME, WHAT IN THE WORLD IS A QUEESTIIIIION!?
(1 vote)
• 1. Yes, I believe so.
2. (from dictionary) a sentence worded or expressed so as to elicit information.
• why does it look like he is about to hack the main frame
• So is evaluating expressions is like you have a variable that stands for a number so you figure out what the value of the variable is first isolate the variable then do the inverse operation?
• Yes, because if you want to isolate the variable in 3+X=7, you could do 3-3+X=7-3, and the 3 and 3 cancel out.
(1 vote)
• Does the letter stand for something in each problem?
• Yes, it stands for the variable’s value.
(1 vote)
• Why do we use a variable. What is its purpose?
(1 vote)
• A variable is used to define an unknown letter and it can be any number. However, as you work towards more advanced math, certain letters represent certain values, such as the letter e, which is Euler's number. I hope this helped!
• How do you do 2(x-7)=4(x-4)
(1 vote)
• 1) Distribute 2 and 4 to eliminate the parentheses: `2x - 14 = 4x - 16`
2) Subtract one of the X terms from both sides. You can pick either one. Let's say you subtract 2x from both sides (this may be a little easier to work with a positive value as a result).
`- 14 = 2x - 16`
3) To isolate the remaining X term, you need to add 16 to both sides: `2 = 2x`
4) Then, complete the problem by dividing both sides by 2. `1 = x`

Hope this helps.