A boat goes 6 km up stream and 7 km down stream in 5 hours. It goes 3 km up stream and 6 km down stream in 3 hours. The speed of the boat in still water is:
Choose the correct choice from the following.
A) 2 km/hr	B) 5.4 km/hr	C) 4.8 km/hr	D) 3.33 km/hr	E) None of these
Explanation:   Let speed of the boat in still water be x km/hr   speed of the stream be y km/hr   then, speed of upstream = (x-y) km/hr   speed of down stream = (x+y) km/hr   6/(x-y) + 7/(x+y) = 5   3/(x-y) + 6/(x+y) = 3   Take 1/(x-y) = a &amp; 1/(x+y) = b   6a + 7b = 5 ---(1)   3a + 6b = 3 ----(2)   (1) -------&gt; 6a + 7b = 5   (2) x 2 ---&gt; 6a + 12b = 6   ------------------------   -5b = -1   b = 1/5   3a + (6/5) = 3   3a = 3 - 6/5   3a = (15 - 6)/5 = 9/5   a = 3/5   3/5 = 1/(x-y)   1/5 = 1/(x+y)   3x - 3y = 5 ----&gt;(iii)   x + y = 5 ----&gt; (iv)   (iv) x 3 ----&gt; 3x + 3y = 15   (iii) -------&gt; 3x - 3y = 5   -----------   6x = 20   x = 20/6 = 10/3 = 3.33   Answer: Option 3.33 km/hr
<result>D</result>
A boat goes 6 km up stream and 7 km down stream in 5 hours. It goes 3 km up stream and 6 km down stream in 3 hours. The speed of the boat in still water is: