A and B are two alloys of gold and copper prepared by mixing metals in the ratio 7 : 2 and 7 : 11 respectively. If equal quantities of the alloys are melted to form a third alloy C, then the ratio of gold and copper in alloy C will be.

$$\eqalign{ & {\text{Gold in C}} \cr & = \left( {\frac{7}{9} + \frac{7}{{18}}} \right)\,{\text{units}} \cr & = \frac{{\text{7}}}{{\text{6}}}\,{\text{units}}{\text{}} \cr & {\text{Copper in C}} \cr & = \left( {\frac{2}{9} + \frac{{11}}{{18}}} \right)\,{\text{units}} \cr & = \frac{5}{6}\,{\text{units}}{\text{}} \cr & \therefore {\text{Gold}}:{\text{Copper}} \cr & = \frac{7}{6}:\frac{5}{6} \cr & = 7:5 \cr} $$