Answer: He will take 53.57 hours in returning the trip
Step-by-step explanation:
Since we have given that
Distance traveled by a cruise ship = 1500 miles
Time taken to travel from London to Athens is given by

Now,
Speed is given by

Now,
if he increase the speed 10 miles per hour
So, new speed becomes

Time taken in returning trip will be

Hence, he will take 53.57 hours in returning the trip.