Chemistry percentage yield question?

A chemist mixed 12.0g of phosphorus with 35.5g of chlorine gas to synthesise phosphorus (III) chloride (phosphorus trichloride). The yield was 42.4g of PCl3. Calculate the percentage yield for this reaction. The equation for the reaction is: 2P(s) + 3Cl2(g) = 2PCl3(l). Please help, my text book is so frustrating and only gives examples whereby there is an excess of a substance or the molar ratios are 1:1.