MAT Exam 2021 Paper

Concept:When a boat moves downstream, the current adds to its speed; upstream, the current reduces its speed.Explanation:Let the distance between $A$ and $B$ be $d$ km.Downstream speed from $A$ to $B$ = $9+3=12$ km/h.Upstream speed from $B$ to $A$ = $9-3=6$ km/h.Total time for both journeys is 3 h.Therefore, $\frac{d}{12}+\frac{d}{6}=3$.Multiplying by 12: $d+2d=36$.So, $3d=36$.Hence, $d=12$ km.Answer:The distance between $A$ and $B$ is $12$ km (Option D).