Fergus Castle is a tourist attraction on a small island off the coast of the mainland. A ferry

Question

service runs between the mainland and the island, and takes 40 minutes to make a single
journey in either direction. There is a turnaround time of 5 minutes at the end of each single
journey. Each day, ferries leave the mainland every 20 minutes, at 5, 25 and 45 minutes past
each hour, starting at 09:05. Ferries leave the island at 15, 35 and 55 minutes past each hour,
starting at 09:15.
What is the minimum number of boats needed?

How do I solve this? I have no idea. Thank you so much in advance.