In a 400 m race, A runs at 16 m/s. A gives B a 16 m start and still beats B by 40 s. What is B’s speed (m/s)?

Introduction / Context:
Starts reduce the distance for one runner. If the faster runner still wins by a time margin, we can deduce the slower runner's average speed from course lengths and finish times.

Given Data / Assumptions:

• A's speed = 16 m/s; A runs full 400 m.
• B gets a 16 m start ⇒ B's course = 400 − 16 = 384 m.
• A beats B by 40 s (B finishes 40 s after A).

Concept / Approach:
Compute A's finish time, then add 40 s to get B's finish time over 384 m, and finally obtain B's speed as distance/time.

Step-by-Step Solution:
A's time = 400 / 16 = 25 s B's time = 25 + 40 = 65 s B's speed = 384 / 65 ≈ 5.9077 m/s ≈ 5.9 m/s

Verification / Alternative check:
With 5.9 m/s, B's time ≈ 65 s; A's is 25 s, confirming the 40 s gap. The 16 m start is correctly accounted for by shortening B's course to 384 m.

Why Other Options Are Wrong:
6 or 8 m/s yield different time gaps; 15 m/s would make B faster than A, contradicting the result that A wins by 40 s.

Common Pitfalls:
Forgetting to reduce B's course by the start; misreading the phrase “beats by 40 s” (it means B's time is 40 s longer).

Final Answer:
5.9 m/s