Simplify the following text for a Grade 10 native English-speaking person.
Original: In 1919 , Ramanujan published a new proof of Bertrand 's postulate ( which had already been proven by Pafnuty Chebyshev ) .
Simplified: In 1919 , Ramanujan published a new proof of Bertrand 's postulate which , as he says , was first proved by Chebyshev .