You have access to an expressway to apply for a job in another city, 300 km away. an interview was scheduled for 11:15 am. you plan to drive at 100km / h and leave at 8:00 am for some time to spare. you drive at the planned speed for the first 100km, after a stretch of the road is under construction and requires you to reduce your speed to 40km / h for 40km. What is the slowest speed you must maintain for the rest of the trip to get time for an interview?

Vm = 128 km / h

Explanation:

The interview is at 11:15 am and he starts a trip at 8 am, the total time he has to arrive is -

ΔT = 3 hours and fifteen minutes = 3.25 hours

The total space he needs to cover is 300 km.

The time he spends on the first 100 kilometers -

V = ΔS / Δt

100 = 100 / Δt1

Δt1 = 1 hour

The time spent in the next 40 km

Vm = ΔS / Δt

40 = 40 / Δt2

Δt2 = 1 hour

The space he has already covered in two hours -

ΔS = 100 + 40 = 140 km

Make total time he has already spent 2 hours, left over -

Δt = 3.25 - 2

Δt = 1.25 hours

The space he needs to travel is missing -

ΔS = 300 - 140

ΔS = 160 km

The average speed he must maintain to get a time -

Vm = 160 / 1.25

Vm = 128 km / h