There are five studios of five different colors. Each studio belongs to a keyboard player of a different nationality. Each of these guys play only one synthesizer, processes its sound with only one processor and listens to the results through only one pair of monitors. The units are all unique, i.e. there are no doubles.
Data:
The English keyboardist has the red studio.
The French keyboardist uses a Roland Jupiter 8.
The Italian guy uses a TC Electronic FireworX.
The green studio is to the left of the black studio.
The owner of the green studio uses an Eventide H-8000.
The keyboardist who uses Genelec 1031 A monitors also has an ARP 2600.
The owner of the yellow studio uses Dynaudio Air-20 monitors.
The keyboardist in the middle studio uses a Lexicon PCM-81.
The American keyboardist has the first studio.
The owner of the Mackie HR-824 is next to the guy who has a Korg MS-20.
The owner of the SCI Prophet 5 is next to the guy who uses the Dynaudio Air-20 monitors.
The keyboardist who uses the Yamaha NS-10 also uses a Manley Massive Passive.
The Swedish guy uses JBL monitors.
The American’s studio is adjacent to the blue one.
The keyboardist who uses the Mackie HR-824 has a neighbor that uses an Avalon AD 2055.
the answer to the riddle: The swedish guy has the Voyager.
This riddle is better known as the Einstein riddle (modified by me of course). He wrote it in the first part of the 20th century and he claimed that 98% of the people cannot solve it. I think the percentage he gave is a little too much as 3 guys who took the time to solve it got it right and one got it wrong.