A person travels from city A to city B. If he travels the entire distance at 40 km/h, he reaches 1 hour late. If he travels the entire distance at 60 km/h, he reaches 1 hour early. He starts at his usual scheduled time. He travels at 40 km/h for the first 1.5 hours. Then, he increases his speed by 50% for the next 1 hour. After that, he takes a 30-minute stoppage. To reach city B exactly at his scheduled time, what must be his speed for the remaining distance?
D
Step-by-Step Solution
Key idea: This is an early/late framework problem combined with piecewise motion and a stoppage. You must first find the total distance and scheduled time, then calculate the remaining distance and time to find the required speed.
Step 1: Find total distance (D) and scheduled time (T).
Let D be the distance.
At 40 km/h, time = D/40 = T + 1.
At 60 km/h, time = D/60 = T - 1.
Subtract the two equations: D/40 - D/60 = 2.
(3D - 2D) / 120 = 2 => D / 120 = 2 => D = 240 km.
Substitute D back: 240 / 40 = T + 1 => 6 = T + 1 => T = 5 hours.
Step 2: Calculate distance and time for the first two segments.
Segment 1: 1.5 hours at 40 km/h. Distance = 1.5 * 40 = 60 km.
Segment 2: Speed increases by 50% -> 40 * 1.5 = 60 km/h.
Time = 1 hour. Distance = 1 * 60 = 60 km.
Total distance covered so far = 60 + 60 = 120 km.
Total time used so far = 1.5 + 1 = 2.5 hours.
Step 3: Account for the stoppage and find remaining requirements.
Stoppage = 30 minutes = 0.5 hours.
Total time used including stoppage = 2.5 + 0.5 = 3 hours.
Time remaining to reach on time = T - 3 = 5 - 3 = 2 hours.
Distance remaining = D - 120 = 240 - 120 = 120 km.
Step 4: Calculate required speed for the remaining distance.
Required speed = Distance remaining / Time remaining = 120 / 2 = 60 km/h.
Answer: 60 km/h