Acceleration and velocity problem.

[kencamarador]

[STRIKE]A boy is running a a constant velocity of 3.0 m/s [E] and passes his father who is standing still. The father immediately starts to chase his son with a constant acceleration of 1.5m/s at the instant his that his son reachers him

How long does it take the father to catch his son?

So to find time. I did

T= vf - vi/aav

Vf is final velocity (3.0-0)
Vi is initial velocity (0)
Aav is average velocity (1.5)

I got 2 Seconds? But the correct answer is 4? How?

 

[TSny]

You found the time when the father has the same velocity as the boy. But you need to find the time that the father has the same position as the boy. In order for the father to catch his son, the father will need to end up running faster than the son.