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. It 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 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.
"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.