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### Course: Grade 7 (VA SOL)>Unit 2

Lesson 1: 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
(17 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.
(40 votes)
• hey guys i just started algebra any tricks becaus this is hard
(13 votes)
• 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.
(16 votes)
• Why is X the most used variable is it because of the x and y axis or some other reason
(6 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
(13 votes)
• You got this guys! Even if it's hard, i believe in you!

Keep going!
(11 votes)
• If t=11, what is the answer?
(5 votes)
• expression: 5t+3

If t=11:

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

Hope this helps!
(8 votes)
• What happens if t is a decimal, a logarithmic function or even worse an exponential function?
(6 votes)
• If "t" is a decimal, you do the same thing you would with a whole number.
(2 votes)
• 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.
(6 votes)
• 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.
(5 votes)
• At , is 5. (1) not the same as 5 x x 1 ?
(4 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.
(9 votes)
• how do i do this with fractions?
(4 votes)
• I've always been curious about PEMDAS. Who created the rule that multiplication and division comes before addition and subtraction? Who established this particular order? Why can't we add before multiplying?
(3 votes)
• It could be to do with the fact that multiplication is like repeated addition, so it tells us to perform the 'repeated additions' before the singular additions.
Or maybe since multiplication/division are operations that build on addition/subtraction.
But good question!
(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.