Question: What is the 2015 population of the city in which Silberturm was located. 
Sentence (from "Frankfurt"): Frankfurt, officially Frankfurt am Main (Literally "Frankfurt on the Main", ] ), is a metropolis and the largest city in the German state of Hesse and the fifth-largest city in Germany, with a 2015 population of 732,688 within its administrative boundaries, and 2.3 million in its urban area.
Relevant evidence: Yes