At what time does the particle change direction?

Question

A particle moves along the y-axis so that its position at time 0 ≤ t ≤ 20 is given by y(t)= 5t - (t^2)/3

Answer 1

5t - t^2/3 = 0

15t - t^2 = 0

t(15 - t) = 0

t=0 or 15-t = 0

t = 0 or t = 15

5(15) - 15^2/3 = 75 - 225/3 = 75 - 75 =0