SOLUTION: Please help me set up this problem: "When Ace Airlines changed to planes that flew 100km/hour faster than its old ones, the time of its 2800 km Dallas-Seattle flight was reduced

Question 427559: Please help me set up this problem:
"When Ace Airlines changed to planes that flew 100km/hour faster than its old ones, the time of its 2800 km Dallas-Seattle flight was reduced by 30 minutes. Find the speed of the new planes."
Answer by mananth(16946) (Show Source): You can put this solution on YOUR website!
original speed = x
increased speed = x+100
...
Original time = 2800/x
Time after speed increase = 2800/(x+100)
2800/x - 2800/(x+100)=1/2
LCD = x(x+100)
multiply by x(x+100)
2800(x+100)-2800*x= 1/2* x(x+100)
2800x+280000-2800x=1/2(x^2+100x)
5600x+560000-5600x= x^2+100x
x^2+100x-560000=0
Find the roots of the equation by quadratic formula

a= 1 ,b= 100 ,c= -560000

b^2-4ac= 10000 + 2240000
b^2-4ac= 2250000

x1=( -100 + 1500 )/ 2
x1= 700
x2=( -100 -1500 ) / 2
x2= -800
Ignore negative value
plane speed = 700 mph
New plane speed = 800 mph

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