Question

It takes a boat 2 hours to travel 22 miles downstream and 4 hours to travel 20 miles upstream. What is the speed of the boat in still water?

Answer 1

Let  b= speed of boat in still water
Let  c= speed of current
Upstream
Speed is ; b-c
d u=(b-c)* t u

Downstream
speed is ; b+c
d d=(b+c)*t d

given:
d u=18
t u=3
d d=24
t d=2


------------------
(1) 18=(b-c)*3
(2) 24=(b+c)*2
and
(1) 18=3b-3c
(2) 24=2b+c
Multiply both sides of (1) by 2
Multiply both sides of (2) by 3

(1) 
(2) 
Add (1) and (2)


And, from (1),
18=3*9-3c
3c=27-18
3c=9
c=3
The speed of the boat in still water is 9 mi/hr
The speed of the current is 3 mi/hr
check:
(1) 18=(9-3)*3
(2) 24=(9+3)*2
and
(1) 18=16*3
18=18
(2) 24=24