SOLUTION: a car covers one -quarter of distance to the next town at 40km/hr,another quater at 50km/hr,and the rest at 90km/hr,find the car's average speed for the entire distance

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Question 490168: a car covers one -quarter of distance to the next town at 40km/hr,another quater at 50km/hr,and the rest at 90km/hr,find the car's average speed for the entire distance
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
For average speed, always use (total distance) / (total time)
Let the entire distance = d km
car's time for 1st +d%2F4+
+t%5B1%5D+=+%28d%2F4%29%2F40+
+t%5B1%5D+=+d%2F160+ hrs
car's time for 2nd +d%2F4+
+t%5B2%5D+=+%28d%2F4%29%2F50+
+t%5B2%5D+=+d%2F200+ hrs
car's time for last +d%2F2+
+t%5B3%5D+=+%28d%2F2%29%2F90+
+t%5B3%5D+=+d%2F180+ hrs
average speed = +d+%2F+%28d%2F160+%2B+d%2F200+%2B+d%2F180%29+
Divide top and bottom by +d
average speed = +1+%2F+%281%2F160+%2B+1%2F200+%2B+1%2F180%29+
Multiply top and bottom by +20+
average speed = +20+%2F+%281%2F8+%2B+1%2F10+%2B+1%2F9%29+
Multiply top and bottom by 360
average speed = +7200+%2F+%2845+%2B+36+%2B+40%29+
average speed = +7200+%2F+121+=+59.5+ km/hr