Let the speed of boat in still water =B
Let the speed of stream in still water =S
Speed of boat in downstream =B+S
Speed of boat in upstream =B-S
B+S=21/2=10.5km/h
B-S=15/2=7.5km/h
Solving after –
B=9km/h and S=1.5km/h
ATQ-
S =1.5 =3km/h
B=9=18 km/h
Now total taken time =
(21/B-S) +(15/B+S)
= (21/15) +(15/21)
=74/35
=2(4/35) hours
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