The speed of a boat in still water is 400\% more than the speed of the stream. The boat covers a total distance of 240 km downstream and 140 km upstream in 15 hours. What is the time taken by the boat to travel 70 km upstream?

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**Solution:**

1. Let the speed of the stream be *x* km/h.

2. The speed of the boat in still water is 400\% more than the speed of the stream, which means:
   
   $$	ext{Speed of boat} = x + 4x = 5x 	ext{ km/h}$$

3. Speed downstream = speed of boat + speed of stream = $5x + x = 6x$ km/h.

4. Speed upstream = speed of boat - speed of stream = $5x - x = 4x$ km/h.

5. Given total time for 240 km downstream and 140 km upstream is 15 hours:
   
   $$\frac{240}{6x} + \frac{140}{4x} = 15$$

6. Simplify the equation:
   
   $$\frac{240}{6x} = \frac{40}{x}$$
   $$\frac{140}{4x} = \frac{35}{x}$$

So,
   
   $$\frac{40}{x} + \frac{35}{x} = 15$$
   $$\frac{75}{x} = 15$$

7. Solving for *x*:
   
   $$x = \frac{75}{15} = 5 	ext{ km/h}$$

8. Speed upstream = $4x = 4 	imes 5 = 20$ km/h.

9. Time taken to cover 70 km upstream:
   
   $$	ext{Time} = \frac{	ext{Distance}}{	ext{Speed}} = \frac{70}{20} = 3.5 	ext{ hours}$$

**Answer:** 3.5 hours

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