A and B run a 100 m race. A runs at 5 km/h and gives B a start of 8 m, yet beats B by 8 seconds. What is B's speed?

Problem restatement
B starts 8 m ahead (needs to run 92 m). A runs 100 m at 5 km/h and finishes 8 s before B. Find B's speed.

Given data

• A's speed = 5 km/h = 5 × 1000 ÷ 3600 = 25/18 m/s ≈ 1.3889 m/s.
• A's distance = 100 m; B's distance = 92 m.
• Finish-time gap = 8 s (B is slower).

Concept/Approach
Compute A's time, then set B's time = A's time + 8 to solve for B's speed.

Step-by-step calculation
t_A = 100 ÷ (25/18) = 100 × 18 ÷ 25 = 72 s t_B = t_A + 8 = 80 s v_B = 92 ÷ 80 = 1.15 m/s Convert to km/h: 1.15 × 3.6 = 4.14 km/h

Verification/Alternative
Check: In 80 s at 1.15 m/s, B runs 92 m; A's 72 s time confirms the 8 s margin.

Common pitfalls

• Letting B run 100 m despite the 8 m start.
• Incorrect unit conversion between m/s and km/h.

Final Answer
4.14 km/h