Think about the reverse problem: a car is still, and it reaches its top speed in 120m, with the same acceleration (compare it to sending people inside a room until it's full, or bringing people outside until it's empty, with 5 people entering/leaving each second).
Your car moves according to [tex]\Delta x = \frac12 a t^2[/tex], where [tex]\Delta x[/tex] is your 120 meters, and a is 5[tex]ms^{-2}[tex]. It's an equation in t, solve for it and take the positive root (why?)
