A boy runs for 10 minutes at a uniform speed of \[\text{9 km }{{\text{h}}^{\text{-1}}}\]. At what speed should he run for the next 20 minutes so that the average speed comes to\[\text{12 km }{{\text{h}}^{\text{-1}}}\]?

• A. \[\text{13}\text{.5 km }{{\text{h}}^{\text{-1}}}\]  
• B. \[\text{10}\text{.2 km }{{\text{h}}^{\text{-1}}}\]
• C.                \[\text{8}\text{.2 km }{{\text{h}}^{\text{-1}}}\]    
• D.        \[\text{7}\text{.72 km }{{\text{h}}^{\text{-1}}}\]