Complex sentence: He taught high school mathematics in Worcester from 1899 to 1920 and then, at age 47, returned in 1920 to graduate study in mathematics at Clark University, receiving a master's degree in 1921.
Split into simpler sentences: He taught high school mathematics in Worcester from 1899 to 1920. At age 47, returned in 1920 to graduate study in mathematics at Clark University, receiving a master's degree in 1921.