The correct statement is:
"By 1950 the population of Pittsburgh was about 305,000; the population of Pittsburgh was about 675,000 in 2010."
However, the numbers in the statement seem to be incorrect for the years mentioned. In reality, by 1950, the population of Pittsburgh was around 676,000, and by 2010, it had dropped to about 305,000.
Thus, the statement, although it captures the concept of population decline, has the population figures reversed.