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.
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A105 sec.
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B100 sec.
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C95 sec.
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D90 sec.
Answer
Correct Answer: 95 sec.
Explanation
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:
B speed = 15 / 5 = 3 m/sDistance B covered when A finishes = 300 − 15 = 285 mCommon time = 285 / 3 = 95 sVerification / 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.