a train speeds past a pole in 20 seconds and passes through a tunnel 500 m long in 40 seconds . its length is :
sol . let the length of the train be x metres and its speed be y m / sec . then , x / y = 20 ⇒ y = x / 20 ∴ ( x + 500 ) / 40 = x / 20 ⇔ x = 500 m . answer c
multiply(500, subtract(const_2, const_1))