A pollution control device reduces the rate of emission of an air pollutant
. . in a car's exhaust by 56 parts per million per hour.
When the device is installed, it will take the car 10 hr to emit the same amount
. . of pollutant that it formerly did in 3 hr.
What was the original rate of emission of the pollutant in the car's exhaust?
Let = parts per million.
Let ppm/hr = engine's original rate of emission of pollutant.
In 3 hours, it emits ppm of pollutant.
With the device installed, the engine's rate is: ppm/hr
In 10 hours, it emits ppm of pollutant.
These two amounts are equal: .
Therefore, the original rate of pollutant emission is ppm/hr.