The current effects the boat and the child equally, so you can ignore it. Just solve the problem as if there is no current.
(black arrows = "Stromversetzung" - I don't know an english word for that )
Google Translate gives "current dislocation", so maybe current vector? Or current velocity?
Typing 'current' into an english-german dictionary gives a number of definitions, one of which is strom
. Translated back to English, we can see that strom
means "river" (we get our word "stream from it").
From this, I'd feel safe inferring that a better translation for Stromversetzung
would be something like "stream's dislocation." Base on that interpretation, I'd say that the black arrows are displacement vectors for the river's current, the red arrow is the displacement vector for where one should aim, and the blue arrow is their sum, ie, where one ends up reaching the child
from wordreference.c0m; but I cannot post links.