I got 13.1 % which is the correct answer.
This was from Nov 2003 / CIE / P1. I copied the question
as i have it here. Anyway thank you for your prompt answer.
Your original post left out "P2O5" in your question, which changes
the question entirely. In future, for all CIE questions, please
link directly to the question online, whenever available.
http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_w03_qp_1.pdf
Working is (as I explained in my previous post)
( 30/100 ) x ( ( (2 x 31) / ( (2 x 31) + (5 x 16) ) ) = 0.131 or
13.1%
No prob, Hoay :)