Two people 400 m apart walk toward each other, each at 4 km/h. A dog runs back and forth between them at 30 km/h until they meet. Its total distance is s = 5x×10² m. Find x.
Physics · Mixed Chapter
JEE Main Practice

Question

Two people 400 m apart walk toward each other, each at 4 km/h. A dog runs back and forth between them at 30 km/h until they meet. Its total distance is s = 5x×10² m. Find x.

G1 Physics · Accepted Answer

Step 1: Their closing speed is 4+4 = 8 km/h. Step 2: Initial separation is 400 m = 0.4 km, so meeting time t = 0.4/8 = 0.05 h. Step 3: Dog covers s = 30×0.05 = 1.5 km = 1500 m. Step 4: 1500 = 5x×100 ⇒ x = 3.