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## Algebra basics

### Course: Algebra basics > Unit 2

Lesson 1: Introduction to variables# 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.

## Want to join the conversation?

- how do i solve if 8 is 90% of n(9 votes)
- 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.(13 votes)

- At0:41, is 5. (1) not the same as 5 x x 1 ?(5 votes)
- 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.(6 votes)

- Why is X the most used variable is it because of the x and y axis or some other reason(4 votes)
- 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(6 votes)

- If t=11, what is the answer?(3 votes)
- expression: 5t+3

If t=11:

(5x11)+3

=55+3

=58

Hope this helps!(1 vote)

- why does it look like he is about to hack the main frame(4 votes)
- 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?(3 votes)
- 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.(2 votes)

- Does the letter stand for something in each problem?(3 votes)
- Yes, it stands for the variable’s value.(2 votes)

- Why do we use a variable. What is its purpose?(2 votes)
- 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!(4 votes)

- 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.(5 votes)

- How do you do 2(x-7)=4(x-4)(2 votes)
- 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.(3 votes)

## Video transcript

A local hospital is holding
a raffle as a fundraiser. The individual cost of
participating in the raffle is given by the following
expression-- 5t plus 3, or 5 times t plus 3,
where t represents the number of tickets
someone purchases. Evaluate the expression when t
is equal to 1, t is equal to 8, and t is equal to 10. So let's first
take the situation where t is equal to 1. Then this expression
right over here becomes-- and I'll use that
same color-- becomes 5 times 1. 5 times 1 plus 3. 5 times 1 plus 3, and we know
from order of operations, you do the multiplication
before you do the addition. So this will be 5
times 1 is 5 plus 3, and then this is
clearly equal to 8. Now let's do it when
t is equal to 8. So when t is equal to 8,
this expression becomes-- and I'll do the same colors
again-- 5 times 8 plus 3. Same color of green. And once again, 5 times 8 is
40, and then we have the plus 3, there so this is equal to 43. And so we have the
last situation, with t is equal to 10. I'll do that in blue. So we have 5 times 10. So 5t is 5 times 10. Instead of a t, put a 10 there. 5 times 10 plus 3. That's a slightly
different shade of green, but I think you get the idea. 5 times 10 is 50. We do 50, and then we're going
to have to add 3 to that, and that is equal to 53. And we're done.