Answer: 14 mph
Explanation:
Let x = the speed of the boat in still water.
Then the speed of the boat relative the bank of the river is u-5 km/hb when it moves upstream, and u+5 when it moves downstream.
We can write the basic equation for travel and distance problems
Time = Distance/Speed in the form
t = 9/x-5 for moving upstream, and
t = 19/x+5 for moving downstream.
Since the left side, t, is the same in both equations, you get a single equation for u
9/x-5 = 19/x+5
To solve it, multiply both sides by the product (x-5)*(x+5) and simplify:
9*(x+5) = 19*(x-5),
9x + 45 = 19x - 95,
45 + 95 = 19x - 9x,
10x = 140,
u = 140/10 = 14 mph.
hope this helps