Definition:Travelling With and Against the Flow Puzzle

Definition
A travelling with and against the flow puzzle is a problem of the form:


 * A body travels a certain distance $d$ through a medium in the same direction as the constant flow of that medium in a given time $t_1$.


 * The same body travels the same distance $d$ in the opposite direction, that is against the flow of the medium, in a given time $t_2$.


 * How long would it take the body to travel $d$ in absence of any such flow?

Variants give:
 * the time taken for the body to travel at the same speed as the medium
 * the time taken for the body to travel either with or against the medium

where the object of the exercise is to work out how long it takes to travel in the opposite direction.