A boat travels a distance of 12 km. The first 4 km along the stream is covered in 15 minutes. The next 8 km is upstream. The downstream speed is twice the upstream speed. What is the average speed for the journey?

Let speed of boat = v and that of stream = u

Speed upstream = v -u

Speed downstream = v +u

As per question, v+u = 2 (v-u)

=> v = 3u

Now, $$time = \frac{distance }{ speed}$$

$$\frac{15}{60} = \frac{4}{ v+u} $$

$$\frac{1}{4} = \frac{4}{ 3u+u} $$

$$\frac{1}{4} = \frac{1}{ u} $$

u = 4 km/h

Then v = 12 km/h

Time taken in upstream journey $$= \frac{8}{ (12 - 4)} = 1 h$$

$$Average speed = \frac{total  distance }{ total  time}$$

$$Average speed = \frac{12 }{ 1+0.25 }$$

=9.6 km/h