1 ship leaves port everyday. After 30 days it returns. How many ships will it see on the way back ?

You have to know the time at sea. For instance, if the voyage there took 2 days, then it stayed in port for 26 days, and two days returning, it saw 4 ships (2 which had left the 2 days before the ship sailed, and 2 which got underway when the returning ship was at sea).