SOLUTION: Two planes make a 1000 mile flight, one flying 75 miles per hour faster than the other. The quicker plane makes the trip 3 hours faster. How long did it take the slower plane to c

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Question 657689: Two planes make a 1000 mile flight, one flying 75 miles per hour faster than the other. The quicker plane makes the trip 3 hours faster. How long did it take the slower plane to complete the flight?
Answer by kevwill(135) About Me  (Show Source):
You can put this solution on YOUR website!
Let s be the speed of the slower plane and s+75 be the speed of the faster plane.
The time it takes the slower plane to make the flight is 1000%2Fs
The time it takes the faster plane to make the flight is 1000%2F%28s%2B75%29
The difference in these two times is 3 hours, so
1000%2Fs+-+1000%2F%28s%2B75%29+=+3

%281000%2As%2B75000%29%2F%28s%5E2%2B75%2As%29+-+%281000%2As%29%2F%28s%5E2%2B75%2As%29+=+3
%281000%2As+%2B+75000+-+1000%2As%29%2F%28s%5E2+%2B+75%2As%29+=+3
75000%2F%28s%5E2+%2B+75%2As%29+=+3
%28s%5E2+%2B+75%2As%29%2A%2875000%2F%28s%5E2+%2B+75%2As%29%29+=+%28s%5E2+%2B+75%2As%29%2A3
75000+=+3%2As%5E2+%2B+225%2As
25000+=+s%5E2+%2B+75%2As
s%5E2+%2B+75s+-+25000+=+0
We can solve for s using the quadratic equation s+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+ with a=1, b=75, and c=-25000
s+=+%28-75+%2B-+sqrt%28+75%5E2-4%2A1%2A%28-25000%29+%29%29%2F%282%2A1%29+
s+=+%28-75+%2B-+sqrt%28+5625%2B100000+%29%29%2F2+
s+=+%28-75+%2B-+sqrt%28+105625+%29%29%2F2+
s+=+%28-75+%2B-+325%29%2F2+
So s = -200 or s = 125. We'll ignore the negative answer because the planes are not flying backward.
So the slower plane is flying at 125 mph and completes the trip in 8 hours.
The faster plane is flying at 200 mph and completes the trip in 5 hours.