<PRE><B>Question 162900</B>
a person traveling 210 km by car drove the first part at 100 km/h and the rest at 80 km/h. If the trip took 2.25 h to complete, what distance was traveled at 100 km/h.
:
Let d = distance traveled at 100 km/h
:
The total distance is given as 210 km, therefore:
(210-x) = distance traveled at 80 km/h
:
Write a time equation: time = {{{distance/speed}}}
:
100km/hr time + 80km/hr time = 2.25 hrs
{{{d/100}}} + {{{((210-d))/80}}} = 2.25
:
Multiply equation by 400 
400*{{{d/100}}} + 400*{{{((210-d))/80}}} = 400(2.25)
:
Cancel the denominator out and you have:
4d + 5(210-d) = 900
:
4d + 1050 - 5d = 900
:
4d - 5d = 900 - 1050
:
-d = -150
:
d = 150 km at 100 km/hr
then
210 - 150 = 60 km at 80 km/hr
:
:
Check solution by finding the times
150/100 = 1.5 hr
60/80  = .75 hrs
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total hr 2.25</PRE>