Final answer:
The cyclist's average speed is 18 miles per hour when they travel 72 miles in 4 hours, calculated by dividing distance by time.
Step-by-step explanation:
The average speed of a cyclist is determined by dividing the total distance traveled by the total time taken. To find the cyclist's average speed in miles per hour for traveling 72 miles in 4 hours, we use the formula:
Speed = Distance ÷ Time
So, we calculate:
Speed = 72 miles ÷ 4 hours = 18 miles/hour
Therefore, the cyclist's average speed is 18 miles per hour (option c).