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Evaluating expressions with two variables: fractions & decimals

CCSS Math: 6.EE.A.2, 6.EE.A.2c

Video transcript

- [Voiceover] Let's see if we can give ourselves some practice evaluating expressions that have two different variables in them So let's see if we can evaluate the expression seven J plus five minus eight K, when J is equal to 0.5 and K is equal to 0.25. So why don't you try to pause the video and evaluate this first before we work through it together. Alright, so if we want to evaluate this thing, everywhere we see a J we want to replace it with a 0.5 and everywhere we see a K we want to replace it with a 0.25, so let's do that. This is going to be seven times, and instead of J I'm gonna put a zero, a 0.5 in there. And then we have plus five minus eight times K. And K we're saying is 0.25. 0.25. So what is this going to be equal to? So if I were to take, if I were to take, and I can color code this, seven times 0.5. Half of 7, that's going to be three and a half, 3.5. Then I have plus five. And then I'm gonna subtract. I am subtracting eight times 0.25. 0.25, this is 1/4. I could rewrite this if I want. 0.25, that's the same thing as 1/4. Eight times 1/4, or another way to think about it is eight divided by four is gonna be equal to two. So this whole thing over here is going to be equal to two. So it's gonna be minus, we have this minus out here, so minus two. And what is this going to be? Well, let's think about it. 3.5 plus five is 8.5, minus two is going to be 6.5. So this is equal to this, is equal to 6.5. Let's do another one of these. Alright. And we'll, just like before, try to work through it on your own before we do it together. Alright, now let's do it together. So over here I have this expression 0.1 M plus eight minus 12 N, when M is equal to 30 and N is equal to 1/4. Alright. So everywhere I see an M I want to replace with a 30. And everywhere I see an N I want to replace with a 1/4. So this is going to be equal to 0.1 times M. M is 30. Times 30 plus eight minus 12 times N, where N is 1/4. N is 1/4. So what is, what is 1/10, This right over here, 0.1, that's the same thing as 1/10 of 30? Well 1/10 of 30, that's going to be three. So this part is three. And we have three plus eight. And then we're gonna have minus. Well what is 12 times 1/4? That's gonna be 12/4, or 12 divided by four, which is going to be equal to three. And now when we evaluate this, so that is equal to this, we have three plus eight minus three. Well, threes are going to, you know positive three, then you're gonna subtract three, and you're just going to be left with, you're just going to be left with an eight. And you're done. This expression when M is equal to 30 and N is equal to 1/4 is equal to eight.