Answer :
10 seconds.
You need to figure out when the height of the bolt reaches 0. Given the equation:
1600 - 16T^2 = 0
Add 16T^2 to both sides
1600 = 16T^2
Divide both sides by 16
100 = T^2
Take the square root of both sides
10 = T
Therefore the bolt will hit the ground after 10 seconds.