Let each equal distance be d km.<br><br>
Time for leg 1 = d ÷ 10 hours.<br>
Time for leg 2 = d ÷ 15 hours.<br>
Time for leg 3 = d ÷ v hours.<br><br>
Total distance = 3d. Total time = d/10 + d/15 + d/v.<br>
Overall average speed = 3d ÷ (d/10 + d/15 + d/v) = 15.<br><br>
Cancel d: 3 ÷ (1/10 + 1/15 + 1/v) = 15.<br>
So 1/10 + 1/15 + 1/v = 3/15 = 1/5.<br><br>
1/10 + 1/15 = 3/30 + 2/30 = 5/30 = 1/6.<br>
1/6 + 1/v = 1/5.<br>
1/v = 1/5 − 1/6 = 6/30 − 5/30 = 1/30.<br>
v = 30 km/h.<br><br>
Final Answer: 30 km/h