# Answer on Mechanics | Relativity Question for Dai

Question #4057

Sally travels by car from one city to another. She drives for 27.0 min at 72.0 km/h, 54.0 min at 33.0 km/h, and 39.0 min at 74.0 km/h, and she spends 8.0 min eating lunch and buying gas.

(a) Determine the average speed for the trip.

(a) Determine the average speed for the trip.

Expert's answer

First, let's admit that

27.0 min = 27/60 h = 9/20 h;

54.0 min = 54/60 h = 9/10 h;

39.0 min = 39/60 h = 13/20 h.

Then, time spent on a trip is

T = 9/20 + 9/10 + 13/20 = 2h.

Total distance is

D = 9/20*72 + 9/10*33 + 13/20*74 = 551/5 km.

So, average speed for the trip is

V = D/T = 551/5/2 = 55.1 km/h.

27.0 min = 27/60 h = 9/20 h;

54.0 min = 54/60 h = 9/10 h;

39.0 min = 39/60 h = 13/20 h.

Then, time spent on a trip is

T = 9/20 + 9/10 + 13/20 = 2h.

Total distance is

D = 9/20*72 + 9/10*33 + 13/20*74 = 551/5 km.

So, average speed for the trip is

V = D/T = 551/5/2 = 55.1 km/h.

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## Comments

satyam24.09.2011 01:58ans 51.65 Km/hr

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