A jet airplane, departing on time and flying between two airports at an average speed of $540 mph, arrives eight minutes late. Departing on time and flying at an average speed of $480 mph, it arrives fifty-three minutes late. What is the number of miles between the two airports?

Work Shown:

Convert the given speeds from "miles per hour" to "miles per minute"

540 mi/hr = (540 mi/hr)*(1 hr/60 min)

540 mi/hr = (540/60) (mi/min)

540 mi/hr = 9 mi/min

480 mi/hr = (480 mi/hr)*(1 hr/60 min)

480 mi/hr = (480/60) (mi/min)

480 mi/hr = 8 mi/min

Let,

If the plane travels 9 mi/min, then,

distance = rate*time

d = 9*x

d/9 = x

x = d/9

Similarly if the plane travels 8 mi/min, then,

d = 8*y

d/8 = y

y = d/8

Subtract the time values y and x

y-x = d/8 - d/9

y-x = 9d/72 - 8d/72

y-x = (9d-8d)/72

y-x = d/72

This difference represents 45 minutes because 53 - 8 = 45. The 53 and 8 are from the fact the plane is late 53 minutes and 8 minutes.

In other words, y-x = 45, so d/72 = 45 which solves to d = 3240 when you multiply both sides by 72.

Extra Info:

If d = 3240, then

x = d/9 = 3240/9 = 360

y = d/8 = 3240/8 = 405

So y-x = 405 - 360 = 45

If the plane travels 9 mi/min (aka 540 mph) then it spends 360 minutes (6 hours) doing so. If the plane travels 8 mi/min (480 mph) then it spends 405 minutes (6 hrs, 45 min) doing so. So this is a more detailed look at why the time gap is 45 minutes.

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