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At 4 km/h, Gaspar is 12 minutes late.
At 16 km/h, Gaspar is 9 minutes early.
Required speed
v = (12 + 9) / (12/16 + 9/4)
= 21 / (3/4 + 9/4)
= 21 / 3
= 7 km/h
Answer: A) 7 km/h

Shortcut Rule:

If speed\( s_1\) makes you a minutes late and speed \(s_2\) makes you b minutes early, then the on-time speed is
\(v = \frac{(a + b) }{ (a/s_2 + b/s_1)}\)
This is a weighted harmonic mean of the two speeds, with the time deviations as weights.
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Correct Answer: A (7 km/h)

GIVEN:
- Speed 1 = 4 km/h (12 mins late)
- Speed 2 = 16 km/h (9 mins early)
Target: Speed needed to arrive exactly on time.

CALCULATION (RATIO METHOD):
- Total Time Difference = 12 - (-9) = 21 mins
- Speed Ratio = 4 : 16 = 1 : 4
- Time Ratio = 4 : 1
- Difference in Time Parts = 4 - 1 = 3 parts = 21 mins => 1 part = 7 mins
- Actual Time at 4 km/h = 4 * 7 = 28 mins
- On-time Scheduled Duration = 28 - 12 = 16 mins

TARGET SPEED:
- Speed * Time = Constant
- Target Speed = 4 km/h * (28 mins / 16 mins) = 7 km/h

Conclusion: Answer Choice A (7 km/h).

kevincan
Gaspar walked from his house to his office at 4 km/h and arrived 12 minutes late. If he had run at 16 km/h, he would have arrived 9 minutes early. At what speed should he have walked or run to arrive exactly on time?

A. 7 km/h
B. 8 km/h
C. 9 km/h
D. 10 km/h
E. 12 km/h
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