Question

Sally travels by car from one city to another. She drives for 20.0 min at 80 km/h, 12 min at 105 km/h, and 45 min at 40 km/h. She spends 13.0 min eating lunch and buying gas.Determine the average speed for the trip.

Answer 1

Answer on Question #46870, Physics, Computational Physics

Question:

Sally travels by car from one city to another. She drives for 20.0 min at 80 km/h, 12 min at 105 km/h, and 45 min at 40 km/h. She spends 13.0 min eating lunch and buying gas. Determine the average speed for the trip.

Answer:

Average speed equals:

$$v = \frac{S}{t}$$

where $S$ - a total distance, $t$ - a total time.

The total time equals:

$$t = 20 \text{ min} + 12 \text{ min} + 45 \text{ min} + 13 \text{ min} = 90 \text{ min} = 1.5 \text{ hours}$$

The total distance equals:

$$S = 20 \text{ min} \cdot 80 \frac{\text{km}}{\text{h}} + 12 \text{ min} \cdot 105 \frac{\text{km}}{\text{h}} + 45 \text{ min} \cdot 40 \frac{\text{km}}{\text{h}} = 77.67 \text{ km}$$

Average speed equals:

$$v = \frac{77.67}{1.5} \frac{\text{km}}{\text{h}} = 51.78 \frac{\text{km}}{\text{h}}$$

Answer: $51.78 \frac{\text{km}}{\text{h}}$

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