✔ 最佳答案
speed for the rest of the journey = v
speed for the first 30 km of the journey = v - 48
30/(v - 48) + 10/v = 1/2
60/(v - 48) + 20/v = 1
60v + 20(v - 48) = (v - 48)v
60v + 20v - 960 = v^2 - 48v
v^2 - 128v + 960 = 0
(v - 120)(v - 8) = 0
v = 120 or v = 8 (rejected)
driving speed for the rest of the journey is 120 km/hr