The Mongolian Revolution of 1921  was a military and political event by which Mongolian revolutionaries, with the assistance of the Soviet Red Army, expelled Russian White Guards from the country, and founded the Mongolian People's Republic in 1924. Although nominally independent, the Mongolian People's Republic was a satellite state of the Soviet Union until 1990. The revolution also ended Chinese occupation over Mongolia, which had existed since 1919. Official Mongolian name of the revolution is "People's Revolution of 1921" or simply "People's Revolution" .
Question: How many years was the Mongolian People's Republic a satellite state of the Soviet Union?
Answer: 66