distance

[demoskopico]

A swimmer practiced in still water and swam at the rate of 4 km/h. When she was swimming upstream, she swam at the rate of 3 km/h. If the swimmer averaged 4 km/h in the still water, upstream, and downstream and swam equal distances upstream and downstream but double in the still water, what was her downstream speed?
 5
 6
 7
 8
 9

I can’t catch neither the wording nor the explanation given.

[kevin]

Suppose speed downstream is v, and distances upstream, downstream and in still water are d,d and 2d kilometers
Total distance is 4d
Total time is d/3 (up) + 2d/4 (still) + d/v (down)


4d/((5d/6) +d/v)=4

v=6

I think that one could simplify the question by reasoning that, excluding the time in still water, the average speed must be 4 km/hr