i would like clarification on this question, thanks!

Question

this is math question which happened to arise in my fortnightly math quiz, was thinking it was 25 but Id like clarification. Thanks! 

A convoy has to travel from P to Q, leaving at a certain time. If the convoy travels at 30 kph it will arrive at Q at 10:00 am, an hour early. If it travels 20 kph it will arrive at noon, an hour late. What should the its speed be to arrive on time?

Answer 1

t represents the time traveled to arrive "on time."

r represents the "correct" speed

 

30(t - 1) = 20(t + 1)

30t - 30 = 20t + 20

10t = 50

t = 5

 

The distance of the trip is 30(5 - 1), 120 km  (You could use 20(5 + 1)).

 

120 = r(5)

24 = r