The ratio between the speeds of two trains is 7 : 8. If the second train runs 400 km in 4 hours, What is the the speed of the first train?  Options
A) 87.0	B) 87.5	C) 87.2	D) 87.1	E) 87.4

Rationale: speed and time are inversely proportional ( when distance is constant ) ⇒ speed ∝ 1 time ( when distance is constant ) here distance is constant and hence speed and time are inversely proportionalspeed ∝ 1 time ⇒ speed 1 speed 2 = time 2 time 1 ⇒ 78 = 4 time 1 ⇒ time 1 = 4 × 87 hr ⇒ speed of the first train = distancetime 1 = 400 ( 4 × 87 ) = 100 × 78 = 12.5 × 7 = 87.5 km / hr answer : b
Answer: B