Answer 1

From the given information,
16t² + 4t = 420
4t² + t – 105 = 0
4t² – 20t + 21t – 105 = 0
4t(t – 5) + 21(t – 5) = 0
(4t + 21)(t – 5) = 0
t = -`-21/4`, 5
But, time cannot be negative.
Thus, the required time taken is 5 seconds.