I have been doing some maths problems including a Riddle by Einstein, and there is a lot clues to the answer for this question:
WHO OWNS THE FISH?
here are the clues :
The Briton lives in the red house.
2. The Swede keeps dogs as pets.
3. The Dane drinks tea.
4. The green house is on the immediate left of the white house.
5. The green house's owner drinks coffee.
6. The owner who has a Nintendo 3DS rears birds.
7. The owner of the yellow house has an Xbox One.
8. The owner living in the centre house drinks milk.
9. The Norwegian lives in the first house.
10.The owner who has a PS4 lives next to the one who keeps cats.
11. The owner who keeps the horse lives next to the one who has an Xbox One.
12. The owner who has a Sony Vita drinks orange juice.
13. The German has a Wii U.
14. The Norwegian lives next to the blue house.
15. The owner who has a PS4 lives next to the one who drinks water.
here is my answer: The german owns the fish as it symbolises that Einstein was born in germany!
