Problem: Find the average velocity of a bird that initially flies towards east at 10 m/s for 100m and then turns around and flies at 20m/s for 15s.

Solution:
To find the average velocity of the bird, we need to calculate the total displacement and total time taken by the bird.

Step 1: Find the displacement of the bird during the first leg of the journey.

Distance = speed × time
Distance = 10 m/s × 100 m
Distance = 1000 m

The bird travels 1000 m towards the east during the first leg of the journey.

Step 2: Find the displacement of the bird during the second leg of the journey.

Distance = speed × time
Distance = 20 m/s × 15 s
Distance = 300 m

The bird travels 300 m towards the west during the second leg of the journey.

Step 3: Find the total displacement of the bird.

Total displacement = 1000 m - 300 m
Total displacement = 700 m towards the east

Step 4: Find the total time taken by the bird.

Total time taken = time taken during the first leg + time taken during the second leg
Total time taken = 100 m/10 m/s + 15 s
Total time taken = 25 s

Step 5: Find the average velocity of the bird.

Average velocity = total displacement ÷ total time taken
Average velocity = 700 m ÷ 25 s
Average velocity = 28 m/s towards the east

Conclusion: The average velocity of the bird is 28 m/s towards the east.