Homework Help: Math

Posted by Emme on Sunday, January 20, 2013 at 2:10am.

A jet flew from Tokyo to Bangkok, a distance of 4800km. On the return trip the speed was decreased by 200km/h. If the difference times of the fights was 2 hours, what was the speed from Bangkok to Tokyo.

Math - Reiny, Sunday, January 20, 2013 at 8:40am
time for 1st flight = 4800/x
time for return flight = 4800/(x-200)

4800/(x-200) - 4800/x = 2
multiply by x(x-200)
4800x - 4800(x-200) = 2x(x-200)
960000 = 2x^2 - 400x
x^2 - 200x - 480000=0
(x - 800)(x + 600) = 0
x = 800 or x = -600 , the negative speed makes no sense

so x = 800 km/h

check:
1st flight = 4800/800 = 6 hrs
2nd flight = 4800/600 = 8 hrs
difference is 2 hours, YEahh!

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