I finally realized Manhattan Prep must have forums, so it would be better to ask there. Sounds obvious, eh?
Why didn't any of you point out that?!
Someone asked about this problem and an employee responded "this one is on our errata page in your student center. It should read 2^n, not 2n. Sorry!"
https://www.manhattanprep.com/gre/fo...17-t10800.html
They have an entire page of errors?!