The speed of three cars are in the ratio of 1:3:5.The ratio among the time taken by these cars to?

d = distance
t = time
v = velocity in d/t

If the distance d equals a constant, then

d = v * t
d = 3v * 1/3 t
d = 5v * 1/5 t

Thus, if the first car needs 1 hour to travel d=100km at speed v=100km/h, the 2nd car will need 20 minutes at speed 300km/h, and the 3rd will need 12 minutes at 500km/h.