the length of the bridge , which a train 145 metres long and travelling at 45 km / hr can cross in 30 seconds , is :
"let the length of the bridge : l the equation now is l + 145 / 12.5 m / s ( 45 km / hr or 12.5 m / s ) = 30 solving , l = 230 m answer : b"
subtract(multiply(divide(multiply(45, speed(const_1000, const_1)), speed(const_3600, const_1)), 30), 145)