Hana's statement is indeed the correct one. How I determined this answer:
The question asks you for the total populations of Cambridge, Massachusetts, and Boston, Massachusetts, altogether. That means you have to add the two given numbers together: 616,535 + 101,355 = 717,890. This, of course, is also what Hana did and got for the answer, therefore her statement is the correct one.
Hope I helped you! :)
