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
Answers
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