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A boat covered 20 km downstream and 28 km upstream. The speed of the boat in still water is 7 km/hr and the speed of the stream is 3 km/hr. Find the time taken to complete the whole journey.

To find the time taken to complete the whole journey, we can use the formula:

Time taken = Distance / Speed

Let's first find the time taken to cover 20 km downstream:

Time taken = 20 km / (7 km/hr + 3 km/hr)

Time taken = 20 km / 10 km/hr

Time taken = 2 hr

Now let's find the time taken to cover 28 km upstream:

Time taken = 28 km / (7 km/hr - 3 km/hr)

Time taken = 28 km / 4 km/hr

Time taken = 7 hr

So the total time taken to complete the whole journey is 2 hr + 7 hr = 9 hr.