Problem: two trains running in opposite directions cross a man standing on the platform in 27 seconds and 17 seconds respectively . if they cross each other in 21 seconds , what is the ratio of their speeds ?
Rationale: "let the speed of the trains be x and y respectively length of train 1 = 27 x length of train 2 = 17 y relative speed = x + y time taken to cross each other = 21 s = ( 27 x + 17 y ) / ( x + y ) = 21 = ( 27 x + 17 y ) / = 21 ( x + y ) = 6 x = 4 y = x / y = 4 / 6 = 2 / 3 i . e 2 : 3 answer : c"
annotated_formula: divide(subtract(27, 21), subtract(21, 17))