(3) Let the speed of currentbe x kmph. ∴ Speed of boat in still water= 6x kmph. ∴ Rate downstream of boat= 6x + x = 7x kmph. Rate upstream = 6x – x = 5x kmph. According to the question,
105
7x
+
105
5x
=12 ⇒
15
x
+
21
x
=12 ⇒
36
x
=12⇒12x=36 ⇒x=
36
12
=3 ∴Speed of boat in still water= 6 × 3 = 18 kmph.