Total time =8 hours Formula Distance = Speed × Time Downstream speed = speed of boat in still water + speed of stream Upstream speed = speed of boat in still water − speed of stream Calculation Let speed of boat and speed of stream be x and y respectively ⇒x+y=20 Now,x= 4y ⇒4y+y=20 ⇒y=4kmph ⇒x=4×4=16kmph Upstreamspeed=16−4=12kmph Let,totaldistancebed Accordingtoquestion,
d
(2×20)
+
d
(2×12)
=8 ⇒
d
40
+
d
24
=8 ⇒3d+5d=8×120 ⇒8d=960 ⇒d=120 ∴ Total distance =120 km