Particle velocity

Question:

The velocity of a particle moving along positive x direction is v = (40 -10t) m/s. Here t is in seconds. At time t = 0, the x coordinate of the particle and initial velocity both are zero.
Find the time when the particle is at a distance of 60 m from origin.

Answer:

v = (40 – 10t)

x = 0 and v = 0 when t = 0, and so v = at and Distance (s) = ½ at².

Substituting for t gives a = (40 – 10t)/t and then s = (20t - 5t²)

For x = +60 m there is no solution but for – 60m we get t = 6s. This means that the object is the other side of the origin.