Answer:
Rotation
Step-by step
because when u flip everything around its called rotation. hope this helps u
If the speed of the stream is x
Time is given by d/r , where r is the rate and d is the distance;
Rate for upstream = (15-x) mph and that of down stream is (15 +x) mph
The time taken will be the same;
Thus; 2/(15-x) = 4/(15+x)
cross multiplying;
30 +2x = 60 -4x
6x = 30
x =5
Therefore the rate of the river is 5 mph
