Answer:
c = speed of current = 3 km/hr
r = speed of boat in still water = 15 km/hr
Step-by-step explanation:
Use the formula distance = time x rate to set up these equations
45 = 2.5 (r+c)
39 = 3.25 (r-c)
(r + c) is the speed of the boat added to the speed of the current, downstream
(r - c) is the speed of the boat minus the speed of the current, upstream
Use one equation to isolate a variable then substitute it into the other equation to solve the system.
39/3.25 = r - c
12 + c = r
45 = 2.5 ( 12 + c + c )
18 = 12 + 2c
6 = 2c
c = 3
Then use the equation 12 + c = r from above
12 + c = r
12 + 3 = 15
r = 15 km/hr
