Multiplying integers word problems (temperature)
About this video
After sunset the temperature drops 4 degrees Celsius per hour for 3 hours. A drop is recorded as negative, so the rate is -4 degrees per hour, and -4 × 3 = -12 degrees Celsius of total change.
You'll be able to
- write a falling temperature as a negative rate.
- multiply a negative rate by a time to find the total change.
- state that multiplying numbers with different signs gives a negative.
Key words
- rate of change
- How much something changes each hour (or other unit of time).
- degrees Celsius
- A unit for measuring temperature.
Transcript
- [Instructor] We are told that once the sun went down, the outside temperature dropped four degrees Celsius per hour over three hours. Increases in temperature are recorded with positive numbers, decreases with negative numbers. What is the total change in temperature over the three hours?
Pause this video and work through that on your own before we do it together. All right, so our rate of change of temperature, we're dropping, so that's a decrease in temperature. So I'll use a negative sign there. So it's negative four degrees Celsius per hour, and then all of this is happening over three hours, so times three hours.
And so what is this going to be equal to? Well, we know that 4 times 3 is 12, but we aren't increasing by 12 degrees. We are decreasing. It's a negative that we are doing, negative four every hour for three hours, so that's why it's negative 12 degrees Celsius.
You might also recognize if you have a negative times a positive or a positive times a negative, you're going to have a negative if you have two different signs when you multiply them. But we're done.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.