<PRE><B>Question 890621</B>
A 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 {{{68/60}}} reduces to {{{17/15}}} hrs
:
Write a time equation; time = dist/speed
:
City time + country time = 1 hr 8min
{{{15/s}}} + {{{72/(2s)}}} = {{{17/15}}}
multiply equation by 30s
30s*{{{15/s}}} + 30s*{{{72/(2s)}}} = 30s*{{{17/15}}}
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
---------------
total = 1.133 hrs which is 6 hr 8 min</PRE>