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be the speed of the boat in still water. If the current is 6 mph, then the speed of the boat upstream must be
and the speed of the boat downstream must be
, or put another way
, the upstream trip can be described as
and the downstream trip can be described as
Solve both equations for d:
Since we have two expressions involving
, the speed of the boat in still water, both equal to
, the distance travelled, set the two expressions equal to each other.
Check the answer:
Still water speed:
Upstream distance equals downstream distance so the answer checks.