Question

A man can swim at 3km/h in still water. A river is 300m wide and flows at 1.5km/h. If the man wants to swim across the river between two points directly opposite at the banks, calculate the
a) direction in which he must head so as to reach his destination
b) time he will take to swim between the points

Answer 1

a) The man must head directly across the river in order to reach his destination.
b) The time he will take to swim between the points is 200 seconds (5 minutes). This is calculated by subtracting the speed of the river (1.5km/h) from the man's swimming speed (3km/h) to get the effective speed of the man (1.5km/h), and then dividing the width of the river (300m) by the effective speed (1.5km/h) to get the time (200 seconds).

Answer 2

AAAaannndd the bot gets it wrong yet again!

he must swim at an angle θ such that sinθ = 1.5/3
so θ = 30° upstream.

his time is thus 0.3/(3 cos30°) = 0.115 hours