A particle is tossed vertically upwards from a point O . At time t seconds later , the position vector of the particle from O is given by r=(12t-5t^2)j m . Find the time when the particle is at least 4m above O .
shouldn't it be done liek this :
12t-5t^2=4 , solving gives t=2 or t=2/5
but the answer given is 1.6 s ??