Multiple choice

A boat takes 16 minutes and 24 minutes to cover a particular distance downstream and upstream respectively. If the speed of the boat in still water is 20 km/h, find the speed of the stream. एक नाव को एक विशेष दूरी बहाव के साथ और बहाव के विपरीत तय करने में क्रमशः 16 मिनट और 24 मिनट लगते हैं। यदि शांत जल में नाव की चाल 20 किमी/घंटे है, तो धारा की चाल ज्ञात कीजिये ।

  1. 4 km/hr./किमी/घं.

  2. 5 km/hr./किमी/घं.

  3. 3 km/hr./किमी/घं.

  4. 2 km/hr./किमी/घं.
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A Correct answer
Explanation

Let distance be D km. Downstream: speed = 20 + s, time = 16/60 hr. Upstream: speed = 20 - s, time = 24/60 hr. D/(20+s) = 16/60 and D/(20-s) = 24/60. Dividing: (20-s)/(20+s) = 16/24 = 2/3. Cross-multiply: 60 - 3s = 40 + 2s, so 5s = 20 and s = 4 km/hr. Option B (5 km/hr) is a distractor from incorrect calculation.

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