As it is stated in this item, in order to determine the amount of time it takes to hit the ground, the equation
-16t² + 1600 = 0
Transpose the terms without variable to the other side of the equation,
-16t² = -1600
Divide the equation by -16.
t² = 100
Solve the value of t by getting the square root of the equation.
t = +/- 10
Thus, it will take the object 10 seconds to reach the ground.