Explanation:
Rate UP stream = ( s - w) = 106 km / 2 hr
( where s = boat speed w = current speed )
(s-w) = 53 km/hr
Rate DOWNstream = ( s+w) = 142 / 2 = 71 kmhr
( s-w) + ( s+w) = 53 + 71 km/hr
2s = 124
s = 62 km /hr then s+w = 71 shows w = 9 km/hr
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