SOLUTION: when ace airlines changed to planes that flew 100 km/h faster than its old ones, the time of its 2800 km Dallas-Seattle flight was reduced by 30 min. Find the speed of the new plan

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Question 29963: when ace airlines changed to planes that flew 100 km/h faster than its old ones, the time of its 2800 km Dallas-Seattle flight was reduced by 30 min. Find the speed of the new planes.
Answer by Paul(988) About Me  (Show Source):
You can put this solution on YOUR website!
let the speed be x (orignal)
increased speed = x+100
30 minutes =0.5hours
Equation:
2800%2F%28x%2B100%29-2800%2Fx=0.5
2800%28%28x%2B100%29-%28x%29%29=0.5%28%28x%29%28x%2B100%29%29
2800%28100%29=0.5%28x%5E2%2B100x%29
280000=0.5x%5E2%2B50x
0.5x%5E2%2B50x-280000=0

x=%28-b%2B-sqrt%28b%5E2-4%2Aa%2Ac%29%29%2F%282%2Aa%29
a=0.5, b=50, c=-280000
x=%28-50%2B-sqrt%2850%5E2-4%2A0.5%2A-280000%29%29%2F%282%2A0.5%29
Simplfy and you get two solutions:
x=-800 and x=700
Remove the negavtive
x=700
700+100=800

Hence, the speed of the new planes is 800km/h
Paul.