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Lesson 4: Substitution & evaluating expressions

# Evaluating an expression with one variable

In this math lesson, we explore the expression 5 t plus 3 to calculate the cost of participating in a hospital raffle fundraiser. By evaluating the expression for different values of t, the number of tickets purchased, we find the costs for buying 1, 8, and 10 tickets, which are 8, 43, and 53 dollars, respectively. 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
• 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.
• hey guys i just started algebra any tricks becaus this is hard
• Yeah, i think that you should try learning simplfying and one step equations first. They are easy because it is what you know just in a basic form. work from there and you will get a basic understanding of algerbra and be able to work forward and go to harder stuff.
• What happens if t is a decimal, a logarithmic function or even worse an exponential function?
• and you can convert decimals to fraction to make the quetion easiar
• 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
• If t=11, what is the answer?
• expression: 5t+3

If t=11:

(5x11)+3
=55+3
=58

Hope this helps!
• 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.
• I love math!... (It's opposite day)
• Paradox: If you say it's opposite day, then it must not be opposite day. Otherwise you'd have said, "It's not opposite day."
But, if you say that, then people will not know that today is opposite day.
• This may sound trite but, what EXACTLY does "evaluate" mean in the context of mathematics?

I ask because I am reviewing the basics, and looking back through my math education it honestly seems like there really isn't a specific definition; it's like the teacher just means "do that thing I showed you in class", almost like little kids sometimes think that "=" means "give me an answer". I have the same problem with "prove". No one ever told me when proving something what I am allowed to assume is true, and some math teachers differ in their opinion on the matter.
• Well i recommend using a definition dictionary (If you don't understand the meaning of a concept) , you can find very useful ones in the internet. Evaluate at least in my experience is to determined something, for example if the sentence says: "Evaluate 2X + 4 when the x value is 5" Well you check the value of the expression when X is five, in this case it will give you fourteen. And that's it, you have determined, checked or evaluated a certain situation.