> To test your understanding, suppose you have a boat that does 20 km/h and
> you sail along a river from A to B and immediately back to A; the distance
> A-B = 20 km.
> 1. If the river stops flowing, how long will your journey take?
> 2. If the river flows at 10 km/h, how long will your journey take?

This is a naive and wrong understanding of absolute time and motion.
Also it disagrees with the SR assertion that the one-way speed of
light is isotropic.