# A boat is able to move through still water at 20 m=s. It makes a round trip to a town 3.0 km upstream. If the river ows at 10 m=s, the time required for this round trip is: a) 100 s b) 200 s c) 300 s d) 350 s e) 400 s

1
by JocelynAquino

2014-06-22T14:35:55+08:00
E. 400s

Solution:

Note: Velocity = Distance/Time

Velocity is ADDED when both the flow of the river and the direction of the boat are the same. Boat is going DOWNSTREAM in this case.

When boat is going downstream:

V₁ + V₂ = Distance/Time
Time = Distance / (V₁ + V₂)
= 3000 meters / ( 20 m/s + 10 m/s)
= 3000/30
Time    = 100 seconds   ....Time going DOWNSTREAM.

Velocity is SUBTRACTED when both the flow of the river and the direction of the boat are the NOT the same. Boat is going UPSTREAM in this case.

When boat is going upstream:

V₁ - V₂ = Distance/Time
Time = Distance / ( V₁ ₋ V₂ )
= 3000 meters / ( 20 m/s - 10 m/s)
= 3000/10
Time  = 300 seconds ........Time going UPSTREAM.

Therefore total time = time downstream + time upstream
= 100 s + 30 s
Round-trip Time    = 400 seconds ........ANSWER