SOLUTION: Justin's boat travels 72 km downstream in 3 hours and it travels 80 km upstream in 4 hours. Find the speed of the boat and the current Thank you!!!

The speed downstream is  = 24 km/h.

It is equal to the sum of the boat speed at still water (U) and the current speed (v):

u + v = 24.    (1)


The speed downstream is  = 20 km/h.

It is equal to the DIFFERENCE of the boat speed at still water (U) and the current speed (v):

u - v = 20.    (2)


Thus you have these two equations for 2 unknowns:

u + v = 24,    (1)
u - v = 20.    (2)


Add the equations. You will get

2u = 24 + 18 = 44  ====>  u =  = 22.


Thus you just found the speed of the the boat in still water.  It is 22 km/h.


Then from (1)  v = 24-u = 24 - 22 = 2 km/h.


Answer.  The speed of the the boat in still water is 22 km/h.

         The speed of the current is 2 km/h.