Difficulty: Medium
Correct Answer: 3 min.
Explanation:
Introduction / Context:
This is a chained comparison problem: two distance advantages relate A–B and A–C speeds; a time gap relates B–C directly. We solve for C’s time using speed ratios and the given 9 s difference.
Given Data / Assumptions:
Concept / Approach:
vA/vB = 880/840 = 22/21 and vA/vC = 880/798 = 440/399. Hence vB/vC = (vA/vC)/(vA/vB) = (440/399)/(22/21) = 140/133. Use the 9 s difference to find absolute times.
Step-by-Step Solution:
Verification / Alternative check:
Plugging back shows B's time is 171 s and C's time 180 s, confirming the 9 s gap.
Why Other Options Are Wrong:
3.25/4 minutes contradict the achieved 9 s gap with the derived ratios.
Common Pitfalls:
Directly treating distance gaps as time gaps; you must use speed ratios first, then time difference.
Final Answer:
3 min.
Discussion & Comments