Question 1068185
 On a windy day, a boat traveled 210 km in 15 hours.
 When travelling against the wind, it took 21 hours for the boat to travel back.
 What was the rate of the boat in still air and what was the velocity of the wind that hindered the speed of the boat? 
:
let s = the normal speed of the boat on a windless day
let w = the rate of the wind
then
(s+w) = the effective speed with the wind
and
(s-w) = the effective speed against the wind
:
Write distance equation for each way. dist = time * speed
15(s+w) = 210
21(s-w) = 210
simplify both equations, divide the first by 15, divide the second by 21
s + w = 14
s - w = 10
-------------addition eliminates w find s
2s + 0 = 24
s = 24/2
s = 12 km/h is the speed of the boat
:
Find the rate of the wind using the  1st simplified equation
12 + w = 14
w = 14 - 12
w = 2 km/hr is the rate of the wind
:
:
Check this in the statement
" On a windy day, a boat traveled 210 km in 15 hours."
15(12 + 2) = 
15(14) = 210 km