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avg speed qs

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avg speed qs

by shikh » Sat Nov 19, 2011 8:19 pm
A boat traveled from Port Wakefield to Port Adelaide, at an average speed of 12 miles per hour. If the return journey took exactly twice as long, what was the average speed for the round trip?

A.4 miles per hour
B.6 miles per hour
C.8 miles per hour
D.9 miles per hour
E.12 miles per hour
C
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by shankar.ashwin » Sat Nov 19, 2011 8:36 pm
Average Speed = Total distance/ total time

Given, speed from Port Wakefield to Port Adelaide = 12mph and return takes twice as long (or)
Speed from Port Adelaide to Port Wakefield = 6mph (half the speed)

Let the distance be '12' miles

So, onward journey would take 1 hr and return 2 hrs, total = 3hrs and distance (2*12 = 24)

Average Speed = 24/3 = 8mph. C IMO
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by vishal chugh » Sun Nov 20, 2011 7:50 pm
speed for the going journey =12;
return journey takes double time, mean speed becomes half, so speed here is =6;
so average speed when distance for going and returning journey is same(2ab)/a+b;
a= speed for going journey;( in this case = 12)
b= speed for returning journey;( in this case = 6)
hence average speed becomes = 2(12*6)/18 = 8;
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