Ratios and rates. Rates are ratios. Any ratio with different units in the numerator and denominator is called a rate. To solve most questions involving rates of all kinds, all we have to do is set up an equation of the form ratio = ratio. Read full transcript
We just match the units on each side. Remember that such an equation, fraction equals fraction or ratio equals ratio, is called a proportion. And if you're not familiar with it, it may be a good idea to go back and look at the video Operations with Proportions. What you can do and can't do with a proportion mathematically.
That video is in the fraction module. Rates are often expressed as so many units per units. So for example, all of these are rates. Many of these are drawn from science. The last two are special kinds, 60 minutes per hour, or 360 degrees per revolution. These are examples of unit conversion.
So these would be examples of things you would actually be expected to know. The other one's you would not be expected to know. But you would be expected to know that there are 16 minutes in an hour. And you could write this as a ratio. We would set this given rate equal to a fraction with the same units in the numerator and the denominator.
So either the problem itself would give you one of the rates. For example one of the blue rates. Or you would know yourself something about the units and the way that the units are related. You set that up to a ratio. And then you set that equal to a ratio on the other side that matches the same units in the numerator and in the denominator.
Here's a practice problem. Pause the video and then we'll talk about this problem. Okay, so this problem gives us a rate. It gives us the rate of 8 grams per hour. So that has grams in the numerator and hours in the denominator. So we're going to set this equal to a fraction on the other side that also has grams in the numerator and hours in the denominator.
We're gonna have 30 grams in the numerator and then we're just gonna have a variable. I'll call it H, in the denominator. That's the unknown number of hours. The first step I'm gonna do to simplify is I'm gonna divide both sides by 2. So I'm gonna divide the 8 by 2, and divide the 30 by 2. If that operation is an unfamiliar operation.
If you're kind of shocked by that, or didn't know that that was a possible thing to do with proportions, I highly recommend that you go back and watch the video, Operations With Proportions. Again, that video is in the fraction module. At this point, we'll cross-multiply, then to get the H by itself, we'll divide by 4. We get H = 15 over 4 hours.
Write this as a mixed numeral, 3 and three-quarters hours. We're asked when, which is actually a clock time. So we started at noon. Three hours later would be 3 PM 3 quarters of an hour is 45 minutes. So that means, it's completely melted at 3:45 pm. Here's another practice problem.
Pause the video and we'll discuss this. A bumblebee's wing flaps 1440 times in 8 seconds. So, that's essentially a ratio. We could say it's 1440 flaps per 8 seconds. How many times does it flap in a minute? Well, first of all, let's simplify that a little bit.
1440 flaps in 8 seconds, I'm gonna divide by 2. That gets me down to 720 over 4. That I'm gonna divide by 2 again. That gets me down to 360 over 2, and then, I'm gonna divide again, and that's gonna get me down to 180 over 1. So, there are 180 flaps in one second.
Well, we want the flaps in a minute, so clearly, what we're gonna have to do is multiply by 60 seconds, because there's 60 seconds in a minute. So the total number of flaps is going to be 180 times 60. We don't actually need a calculator for this. Let's think about this. Let's drop those 0s and make things a bit simpler.
If we're doing 18 times 6. Well one way to think about this, the 18 is (10 + 8). Well I can do 6 times 10, that's 60. I can do 6 times 8 that's 48. I can add those two, that's 108. So now we're just gonna to add the two zeros and so what we get is a product of 10,800.
And that is the number of flaps in a minute. Here's another practice problem. Pause the video and then we'll talk about this. Okay, this is very tricky. We have a couple rates. We have the 20 grams per centimeter.
We have the $50 per gram. And then we have this initial starting amount, starting volume, which is 2 centimeters cubed. So first thing I'm gonna do is I'm gonna start there, I'm gonna multiply those out and then I get 8 cubic centimeters. Well I wanna multiply that by a rate, so those cubic centimeters cancel.
So I'm gonna multiply it by that first rate, 20 grams per cubic centimeter. That way the cubic centimeters will cancel. Then I'd be left with grams. Now I wanna multiply something, so the grams cancel. If I multiply now by $50 per gram, then the grams cancel and I'll be left with units of dollar.
And that's what we're looking for. We're looking for the price. So, I'll point out here. This is actually very easy now because 20 times 50 is just 1,000 times 8 is 8,000, so that would be worth $8,000. In summary, when you see a problem with rates, remember you can set up proportions.
Always remember to make sure that the units of the numerator and denominators match.