let

the speed of the second car is: v

the speed of the first car is 20% less than the second: v - (20/100)v = v - 0.20v = 0.80v

d = the distance they travel

since v = d/t => t = d/v

the time of the first car is: d/(0.80v)

the time of the second car is d/v

the difference in the time is

(d/(0.80v) - d/v) = (d/v)(1/0.80 - 1) = (d/v)1/0.20 = 0.25(d/v)

the second car needs 0.25 or 25% more time then the first car to travel the same distance.