Correct Answer - Option 5 : 210 km
Speed of first boat from point A to B in downsream direction = 21 + 7 = 28 km/hr
Speed of first boat from point B to A(upstream) = 21 – 7 = 14 km/hr
Let the distance between A to B = D
Total time taken to complete this distance by first boat
⇒ (D/28) + (D/14) = 3D/28
Speed of second boat from point B to A in upstream direction = 28 – 7 = 21 km/hr
Speed of second boat from point A to B(downstream) = 28 + 7 = 35 km/hr
Total time taken to complete this distance by second boat
⇒ (D/21) + (D/35) = 8D/105
According to question,
(3D/28) – (8D/105) = 6.5
⇒ (45D – 32D)/420 = 6.5
⇒ D = 210
∴ The distance between points A and B is 210 km