CHEMISTRY HELP

[fishbiscuit]

I'm doing a past HSC question from the 2009 paper, question 26 about electrochemistry.
I did a)
I did b), and checked the HSC sample answers and got the same answer as them but they did their working out differently. The question involved of working out the standard cell potential (Pb is oxidising and Fe3+ is reducing).
My working out is:
E = 0.77 + 0.13  (because Fe3+ is reducing so V would just be 0.77, and Pb is oxidising so the V would be + 0.13 instead of -0.13)
   = + 0.90 V
Their working out is:
E = 0.77 - 0.13
   = + 0.90 V

?
0.77 - 0.13 is not 0.90, so how did they get to that answer? Can someone explain to me how the maths work around these, thank you !

[RuiAce]

Are you sure that they didn't just make a typo?

[fishbiscuit]

That's what I thought but then isn't it highly unlikely for them to have a typo in their working out...?

[RuiAce]

That's what I thought but then isn't it highly unlikely for them to have a typo in their working out...?

Nope

As in, it's probably unlikely, but not highly unlikely enough to the point you just disregard the possibility of a typo altogether.