A cable-car carriage of mass m m\,m accelerates at a a\,a when the resultant force on it is FFF. After more passengers board, the resultant force is changed to 3F 3F\,3F and the carriage now accelerates at 2a2a2a. What is its new mass?
0.67m0.67m0.67m
0.5m0.5m0.5m
1.5m1.5m1.5m
6m6m6m