[Serena]

I having problems with a problem from the Fundamentals of Analytical Chemistry 8th ed, Skoog, West, Holler, Crouch.

My answer for a can't be correct, but I tried several different ways to do the problem, but still come up with the wrong answer.  So what am I missing?

12.11   The aluminum in a 0.910g sample of impure ammonium aluminum sulfate was precipitated with aqueous ammonia as the
                hydrous Al2O3•xH2O.  The precipitate was filtered and ignited at 1000ºC to give anhydrous Al2O3, which weighed
                 0.2001g.  Express the results of this analysis in terms of
(a)   % NH4Al(SO4)2
                2 NH4Al(SO4)2 ? Al2O3
                Mass NH4Al(SO4)2  = mass Al2O3 * (2 *fw NH4Al(SO4)2 / fw Al2O3)
                mass NH4Al(SO4)2 = 0.2001g (2*237.15/101.96)
                mass NH4Al(SO4)2 = 0.9308
                % NH4Al(SO4)2  = mass NH4Al(SO4)2 / mass sample * 100%
                % NH4Al(SO4)2  =  0.9308/.910 * 100% = 102.29% 
(b)   % Al2O3
                % Al2O3= 0.2001g/.910g * 100% = 21.99%
(c)   % Al
                Mass Al = mass Al2O3 * (2 *Al / fw Al2O3)
                mass Al = 0.2001g * (2*26.98/101.96)
                mass Al = 0.1059g
                % Al = 0.1059/.910 * 100% = 11.64%

[Borek]

My answer for a can't be correct

Answer is correct, book is wrong. It happens.