It takes [[$ 1 \;\text {h} 15 \;\text {min} $]] for the train to travel between two stations at a speed of [[$ 100 \;\text {km/h} $]]. A quarter of the time would be saved if the train traveled at a speed of [[$ 125\;\text {km/h} $]]. Show by calculating whether speed and time are directly or inversely proportional.



Answer: Speed and time are directly inversely proportional.