In 2016 revision notes, I think there's a small error in Qus 1 of September 2013, Ifoa. It should be Z×k/n + (1-Z)×[(a-1)/(a+b-2)] in last line. Please see in the pic below. Thanks
You are absolutely right. I am so sorry for the error. I'll get this in the corrections document and fix it for the next version. Thank you so much for letting us know and apologies for the inconvenience caused.