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### Course: Math (NSDC) - English>Unit 8

Lesson 4: Evaluating expressions

# Example: Evaluating expressions with 2 variables

Evaluating Expressions with Two Variables. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

• how many sign are there that mean to multiply.
• (5)(4) - parentheses
5*4 - the asterisk
5·4 - the center dot
5x4 - the classic primary school symbol for multiplication
5y - a number and a variable together signifies multiplication

As you get further along in math, you will use mostly the first one and the last one.
• where did you you get the parentheses
• The parenthesis were added to show that 10 and -4 are multiplied together. The "why aren't we using the multiplication sign?" video probably explains this better, but basically "10 x -4" can be confusing because the letter x is often used for variable. So, to avoid confusion, you can write "10(-4)" instead of "10 x -4" to show that 10 and -4 should be multiplied. Hope that helps.
• For some reason, I'm having a hard time with fractions being multiplied. How do I handle fractions in all this?

Example: 3/7_r_ + 5/8_s_ when r_ = 14 and _s = 8
From this, I SOMEHOW pulled 69.5 when the answer was 11. How do I work with fractions, here? Haha!
• If it helps, I'll go through that problem for you.

We're going to evaluate `3/7r + 5/8s when r = 14 and s = 8`.

So then `3/7r + 5/8s` becomes `3/7(14) + 5/8(8)`

To do 3/7 times 14, we can write straight 14 as the fraction 14/1. Then an easy way to write two fractions to multiply them is `numerator • numerator/ denominator • denominator`.

So `3/7 • 14/1` becomes `42/7` (3 • 14 = 42 and 7 • 1 = 7)

To do the same to the second fraction, we'll multiply `5/8 • 8/1`. This makes `40/8` (5 • 8 = 40 and 8 • 1 = 8)

The problem is now simplified to `42/7 + 40/8`. This can be simplified further to `6 + 5` (7 can go into 42 6 times evenly and 8 can go into 40 5 times evenly.)

And your answer: `6 + 5 = 11`

I have no idea how you got 69.5, but I tend to do the same thing so I won't judge :). Hopefully this example can help you with fractions in the future.
• Can someone explain how to multiply a fraction by a whole number? I came across this in a quiz.

It said: Evaluatie 1/4c + 3d when c=6 and d=7

when i multiply 1/4 with 6 I get 1 whole number and 2/4... How do I put that in a normal number, and not a fraction?

In the quiz 1/4 times 6 equals 1.5 - i don't get how…
Is 2/4 just a half?
• You get 1/4 out of 6, which number x times 4 will make 6? 1.5, + d is 7 so 3x7 = 21, 21 + 1.5 = 22.5
• I find Khan Academy very helpful. I appreciate what Sal is doing.

Is there a reason why you repeat the same thing multiple times?
"10 times....10 times..."
"Minus 8....Minus 8"
• Sal repeats himself while he takes the times to write what he's saying.
• How would do you convert a fraction to a decimal
• How can you do that if some of the integers are decimals or fractions
• why does 7 have to get multiplied by 10 and then squared
(1 vote)
• 7 does not get multiplied by 10, here is the example in steps....

a^2 + 10b - 8 = ? a = 7 b = -4
Rewrite
7^2 + 10 x -4 - 8 = ?
7^2 is the same thing as 7 x 7, so this equals 49
10 x -4 = -40,
Now you can solve it.
49 + -40 - 8 = ?
49 + -40 = 9
9 - 8 = 1
Remember that exponents come first before x or -.