I enjoy the puzzles sometimes posted on this forum, so I'd like to share one of my own. It involves a very simple analysis, but still provides some interesting insight. I'll use a numerical example (instead of symbolical) because some people like to work with numbers rather than symbols.

Peter lives on the shore of a large lake and drives his motorboat from his home (point A) to the other shore (point B) every day to get supplies, and then back home. Points A and B are 4 miles apart, the water of the lake is exactly quiessent (no currents) and Peter's boat has a constant speed of 16 miles per hour.

His brother James lives on the shore of a river and drives his motorboat from his home (point C) downstream to point D every day to get supplies, and then back home. Points C (upstream) and D (downstream) are 4 miles apart. The river water has a uniform speed of 8 miles per hour, and James' boat is identical to Peter's, i.e. it makes 16 miles per hour, relative to the water.

Both guys commute the same distance to get supplies and get back home (from A to B to A, or from C to D to C, respectively). Their boats make the same speed. However, unlike Peter, James has to deal with the river current, which works with him or against him, depending on the direction.

The question: Whose commute takes a longer time (or are they the same)?

Neglect secondary effects such as air resistance, the effect of the supplies' weight, assumed personalities of James and Peter, or anything else that wasn't mentioned.

The math is really easy and I have no doubt that anyone can do it. I am more interested in the interpretation of the result. Is it intuitive/counter-intuitive?