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b- velocity of boat
w- velocity of water/ bottle
d- distance travel by boat from bridge upto point it travel upsteam( for 20 min)

d=(b-w)*20/60 km = (b-w)/3

time taken by bottle to travel 1km = time taken by boat to travel (distance 'd' upsteam+ distance 'd' downsteam+ 1 km downsteam)
(1/w)= d/(b-w) + d/(b+w) + 1/(b+w)
(b-w)(b+w)= (db+dw+db-dw+b-w)*w
b^2-w^2=(2db+b-w)*w
b^2-w^2= 2dbw+bw-w^2
b=w(2d+1)
resolve equation and put value of d
w=1.5 km/hr
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