Einstein penned the letter in reply to a two-page questionnaire submitted by Arthur Converse, a science teacher from Iowa, concerning electrostatic theory and special relativity.
The letter with an opening price of USD 15,000 was bought by an unidentified person.
The letter had been in the Converse family's possession for many years, said Sam Heller, a spokesman for the Nate D Sanders Auctions auctions that sold the letter.
The letter - whose envelope indicated it was sent from 'Room 115' of the Institute for Advanced Study in Princeton, New Jersey, on September 7, 1953 seems to answer questions about Einstein's theory of relativity and how to reconcile the theory with experiments, 'Live Science' reported.
Converse was a science teacher and had sent Einstein a questionnaire regarding experiments with electroscopes, which are charge-measuring instruments.
The famous physicist also wrote an opening page that says one should expect to measure a potential difference, a voltage, whose level depends on the distance from the Earth's surface.
Einstein wrote that the only assumption one needs to make is that there is some difference in the charge of the surface of the Earth and the upper atmosphere and that the electroscope, in this case, would behave as though it were inside a charged sphere.
In response, Einstein said there is no problem with explaining an experimental result with "standard electrostatic theory."
"It is clear to the reader [that] Einstein was interested in Converse's questions and generous in his considerate response," said Nate Sanders, owner of Nate D Sanders Auctions.
"These documents provide a rare glimpse into Einstein's generous nature and willingness to help teachers better understand and communicate his theories," Nate said.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)