A delivery van and a motorcycle travel the same route of length \(s\) miles. The van travels at a constant speed of \(v\,\text{mph}\), and the motorcycle travels \(15\,\text{mph}\) faster.
a) Write an expression for the difference \(\Delta t\), in hours, between their travel times in terms of \(s\) and \(v\).
b) Explain without calculating how \(\Delta t\) changes if the route length is doubled while both speeds remain the same.
c) Find \(s\) if the van travels at \(45\,\text{mph}\) and the motorcycle arrives \(20\) minutes earlier.
Hints
- Subtract the motorcycle's travel time from the van's travel time.
- Think about how multiplying the same distance by \(2\) affects each travel time.
- Convert \(20\) minutes to hours before writing the equation for part c).
- Clear the denominators to solve for \(s\).
Solution
a) The van's travel time is \(\frac{s}{v}\), and the motorcycle's travel time is \(\frac{s}{v + 15}\). Therefore, \(\Delta t = \frac{s}{v} - \frac{s}{v + 15}\).
b) If \(s\) is doubled while both speeds stay fixed, each travel time doubles. Their difference therefore doubles as well.
c) The speeds are \(45\,\text{mph}\) and \(60\,\text{mph}\), and \(20\) minutes is \(\frac{1}{3}\) hour. Write \(\frac{s}{45} - \frac{s}{60} = \frac{1}{3}\). Multiplying by \(180\) gives \(4s - 3s = 60\), so \(s = 60\).
Answer
a) \(\Delta t = \frac{s}{v} - \frac{s}{v + 15}\)
b) The time difference doubles.
c) The route is \(60\) miles long.