You must go 45 miles per hour to reach the longest distance.
Number of hours the car can run
The given parameters are:
- Speed = 60 miles per hour
- Time = 5 hours
When the speed increases by 1 mile per hour, the time reduces by 10 minutes.
This means that:
When
Speed = (60 + x) miles per hour
The time is:
Time = 5 hours - 10 minutes * x
Convert minutes to hour
Time = 5 hours - 1/6 * x hour
This gives
Time = 5 - x/6 hour
Hence, the car can run for (5 - x/6) hours
The distance for 1 tank of gas
We have:
- Speed = (60 + x) miles per hour
The distance is calculated using
Distance = Speed * Time
This gives
Distance = (60 + x)(5 - x/6)
Expand
Distance = 300 - 10x + 5x - x^2/6
Evaluate the like terms
Distance = 300 - 5x - x^2/6
Hence, the distance for 1 tank of gas is 300 - 5x - x^2/6 miles
The longest distance
We have:
D = 300 - 5x - x^2/6
Differentiate
D' = -5 - x/3
Set to 0
-x/3 = 5
Solve for x
x = -15
Substitute x = -15 in D = 300 - 5x - x^2/6
D = 300 - 5(-15) - (-15)^2/6
Evaluate
D = 337.5
Hence, the longest distance is 337.5 miles
The speed for the longest distance
We have:
x = -15
Substitute x = -15 in Time = (5 - x/6) hours
Time = (5 + 15/6) hours
Evaluate
Time = 7.5 hours
The speed is calculated as:
Speed = Distance/Time
This gives
Speed = 337.5/7.5
Evaluate
Speed = 45
Hence, you must go 45 miles per hour to reach the longest distance.
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