Race timing and lead: In a 300 m race, runner A defeats runner B by 15 m (i.e., when A finishes, B is 15 m behind) and this deficit corresponds to 5 s for B. Find the total time A takes to complete the 300 m race.

Introduction / Context:
We are given a 300 m race where A finishes first. At A’s finish, B is 15 m short of the finish line, and that 15 m takes B exactly 5 s. The task is to compute A’s finish time over 300 m. This tests relative speed and race timing interpretation.

Given Data / Assumptions:

• Total distance for A = 300 m
• B is 15 m behind when A finishes
• For B, 15 m takes 5 s → B's speed = 3 m/s

Concept / Approach:
At the instant A finishes, B has covered 285 m. If we know B's speed, then the shared race time equals the time B took to cover 285 m. That common time equals A's finish time.

Step-by-Step Solution:

Verification / Alternative check:
In 95 s, B covers 95 * 3 = 285 m, exactly 15 m short, matching the statement.

Why Other Options Are Wrong:
90 s, 100 s, 105 s would imply different B deficits than 15 m, contradicting the given 5 s for 15 m.

Common Pitfalls:
Using 300 m with B's speed directly; the common time corresponds to B’s 285 m, not 300 m.

Final Answer:
95 sec.