Sophie bikes s miles per hour. Her friend Rana is slower. Rana bikes r miles per hour. Yesterday, Sophie biked for t hours. How many miles was Sophie's route?
Sophie's distance = speed x time
Sophie biked s miles per hour for t hours, so her distance is:
distance = s x t
Rana's speed is r miles per hour, but we don't know how long she biked for, so we can't calculate her distance.
Therefore, Sophie's route was s x t miles long.