Suppose the temperature outside drops two degrees Celsius each hour. Determine the change in temperature in nine hours.
How do we mathematically represent a drop in temperature? The temperature is going down. And when that happens, we don’t want to use a positive number. We want to use a negative number. The temperature goes down two degrees and we represent that with the integer negative two. This pattern continues for nine hours. That means it’s going to drop by two nine times.
Multiplying a negative by a positive means the final outcome will be a negative. Two times nine equals 18. We remember that we’re dealing with Celsius — two degrees Celsius. And that means that our final answer will also be in degrees Celsius.
After nine hours of this pattern of dropping two degrees each hour, we would represent the change in temperature by negative 18 degrees Celsius.