50 mL of a 2.5 M solution of ammonium phosphate is added to 100 mL of 1.5 M solution of calcium nitrate. At the end of the reaction 11.2 grams of solid product were obtained. Whats the % yield of this reaction?

%yield = (actual yield/theoretical yield)*100 = ??

You find the theoretical yield by stoichiometry. Just follow the steps in this example.
http://www.jiskha.com/science/chemistry/stoichiometry.html