In order to assess which method of transportation is faster, we will look at the time they take, since the distance is the same for both. In order to compare the times, we must make the units that they are both measured in consistent. We must either convert 18.5 minutes into hour or 0.5 hour into minutes. We will go with the latter as it is easier:
0.5 hours = 0.5 hours x 60 minutes per hour = 30 minutes
Therefore, the public transport takes them to the mall faster, since it takes only 18.5 minutes, than cycling to the mall does, which takes 30 minutes.
