Here's the formula you want to use to solve this:
Distance = (1/2) · (acceleration) · (time²)
We know the distance to the ground and the acceleration of gravity, so let's pluggum in:
. . . . . 125 m = (1/2) (9.8 m/s²) (time²)
Divide each side by (4.9 m/s²):
. . . . . (125 m) / (4.9 m/s²) = time²
. . . . . 25.51 s² = time²
Square-root each side:
5.05 seconds = time
