Applying signed number addition to money situations: writing equations
About this video
A community garden fund has $500, the gardener spends g dollars, and $380 is left. The video writes 500 − g = 380 and matches it to the choice 500 + (−g) = 380, explaining why adding a negative is the same as subtracting.
You'll be able to
- write an equation for a money situation where money is spent.
- explain why 500 + (−g) means the same as 500 − g.
- rule out equations that don't match a situation.
Key words
- Equation
- A maths sentence saying two expressions are equal.
- Negative number
- A number less than zero; here −g shows money going out.
- Variable
- A letter, like g, that stands for an unknown amount.
Transcript
- [Narrator] We're told that a community garden has $500 in its fund; later, the gardener for the fund spends some money, g, on tools and supplies. After the purchase, the fund has $380 remaining. Which equation could represent this situation? So pause this video and have a go with that.
Okay, now let's do this together. So I'm not even gonna look at the choices. I'm just gonna see if I can describe what happened with an equation, and see if any of these match that, or if they're close. So they start with $500 in their fund. So I'll write 500 right over here.
Then the gardener for the fund spends g. So we'll subtract g from that. After the purchase, the fund has $380 remaining. So we started with $500, We subtracted g dollars, and then we're left with $380. So this would be my equation. Let's see if any of these have it described the exact same way.
So choice A is not what I this said. Somehow we have a negative $500, and we're adding g. And I could multiply both sides of this equation by negative 1, and the left side would look like this, but on the right side would be negative 380. So I do not like this one.
$500 plus g. This would imply that the gardener's adding g dollars into the fund, whilst the opposite, they're taking it out. So I don't like that one. $500 plus negative g is equal to 380. Yeah, that's essentially what I did. Plus negative g is the same thing as subtracting g, and spends is the key word for why we are subtracting here.
So I like choice C. Now, last but not least, when we look at choice D, $380 plus negative g is equal to 500. It would be true if we were to manipulate this equation right over here, that if we added g to both sides, we would get $380 plus positive g is equal to 500, but not plus negative g.
So I don't like that choice either.
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