rupa Reddy
0

Put h = 135:

⇒ 135 = 60t − 5t2

⇒ 5t2 − 60t + 135 = 0⇒ t2 − 12t + 27 = 0

⇒ (t − 3)(t − 9) = 0⇒ t = 3 or 9


Thus its height is 135 meters after 3 seconds and again after 9 seconds.

(i.e. once on the way up and once on the way down after it has reached its maximum height)