Downstream and back at given boat and stream speeds: total time (or stream speed from it)

This type of question, asked 3 times: 10 Sep 2024 S3, 18 Sep 2024 S3, 23 Sep 2024 S2. All 3 questions below, oldest first.

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Question 1
10 Sep 2024 S3
62 $The speed of a boat in still water is 18 km/h and the speed of the stream is 3 km/h. How much time (in hours) will it take to cover a distance of 105 km in downstream and in coming back?$
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Question 2
18 Sep 2024 S3
66 $A man can row a distance of 6 km in 1 hour in still water. He can row the same distance in 45 minutes with the current. Find the total time taken by him to row a distance of 16 km with the current and to return to the starting point.$
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Question 3
23 Sep 2024 S2
59 $A motorboat, whose speed in 15 km/h in still water goes 50 km downstream and comes back in a total of 7 hours 30 minutes. The speed of the stream (in km/h) is:$
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Question 1