3.2. A man goes downstream with a b...
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3.2. A man goes downstream with a boat to some destination and returns upstream to his origi- nal place in 5 hours. If the speed of the boat in still water and the stream are 10 km/hr and 4 km/hr respectively, the distance of the destina- tion from the starting place is (1) 16 km (2) 18 km (3) 21 km (4) 25 km

11th - 12th Class
Maths
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sol. (3) Let the distance of the destination from the starting point ( =x mathrm{km} ) Rate downstream ( =(10+4) mathrm{kmph}=14 mathrm{kmph} ) Rate upstream ( =(10-4) mathrm{kmph}=6 mathrm{kmph} ) According to the question. ( Rightarrow frac{x}{14}+frac{x}{6}=5 ) ( Rightarrow frac{3 x+7 x}{42}=5 ) ( Rightarrow 10 x=42 times 5 ) ( Rightarrow x=frac{42 times 5}{10}=21 mathrm{km} )
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