Question 1112497
The distance between two islands A and B is 36 km.
 The sea current flows steadily such that a boat can gain a speed of x km/h when it sails from A to B.
 Mr. Chan sails a boat which takes 1.5 hours to travel from A to B.
 and then 2 hours from B to A.
 Find the speed of the boat when there is no sea current.  
:
let s = the speed of the boat with no current
then
(s+x) = effective speed with the current
and
(s-x) = effective speed against
:
Write a distance equation for each way. dist = time * speed
1.5(s+x) = 36
2(s-x) = 36
simplify both equations divide the first by 1.5, divide the second by 2
s + x = 24
s - x = 18
---------------Addition eliminates x, find s
2s + 0 = 42
s = 42/2
s = 21 km/hr is the speed of the boat in still water