Question

A car travels for 15km in a city at a certain speed.Outside the city it travels 72km at twice it's former speed.If the total travelling time is 1h 18min .find the average speed in the city

Answer 1

Step-by-step explanation:

car travel 15KM in a city at a certain speed.

outside the city it travel 72KM at twice the former speed.

if the total traveling time is 1h 8min, find the average speed in the city

:

let s = speed in the city

then 2s = speed in the country

:

change 1 hr 8 min to 68%2F60 reduces to 17%2F15 hrs

:

Write a time equation; time = dist/speed

:

City time + country time = 1 hr 8min

15%2Fs + 72%2F%282s%29 = 17%2F15

multiply equation by 30s

30s*15%2Fs + 30s*72%2F%282s%29 = 30s*17%2F15

cancel the denominators and you have

30(15) + 15(72) = 2s(17)

450 + 1080 = 34s

1530 = 34s

s = 1530/34

s = 45 km/h in the city

:

:

Check this by finding the time at each speed, (90 km/h in the country)

15/45 = .333 hrs

72/90 = .8 hrs

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total = 1.133 hrs which is 6 hr 8 min