Skip to Main Content

Ratios and Rates


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 to set up an equation of the form ratio = ratio. We just much the units on each side.

Remember that such and equation, fraction equals fraction or ratio = 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 a special kind, 60 minutes per hour or 360 degrees per revolution, these are examples of unit conversions. So these would be examples of things you would actually be expected to know.

The other ones you would not be expected to know. But you would be expected to know that there are 60 minutes in an hour, and you can 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 in the way that the units are related. You set that up as a ratio, and then you set that equal to a fraction on the other side that matches the same units in the numerator and 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 were gonna 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 were just gonna have 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 get 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 did know that was 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 model.

At this point will 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. Three-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 1,440 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, 1,440 flaps in 8 seconds, I'm gonna divide by 2, that gets me down to 720 over 4.

Then I'm gonna divide by 2 again, that gets me down to 360 over 2. And then I'm gonna divide again, that's gonna get me down to 180 over 1, so there are 180 flaps in 1 second. Well, we want the flaps in a minute, so clearly what we're gonna have to do is multiply by 60 seconds, cuz there are 60 seconds in a minute. So the total number of flaps is gonna be 180 times 60.

We don't actually need a calculator for this. Let's think about this, let's drop the zeros and make things a bit simpler. If we're doing 18 * 6, well one way to think about this, the 18 Is 10 + 8. Well, I can do 6 times 10, thats 60, I can do 6 times 8, thats 48, I can add those two, thats 108. So now were just gonna 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, this 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 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 easy now, because 20 time 50 is just 1,000 times 8 is 8,000, so that would be worth $8,000. In summary, when you see problems with rates, remember you can set up proportions. Always remember to make sure that the units of the numerator and denominators match.

Read full transcript