A10-g penny is dropped from a building that is 125 m high. the penny is initially at rest. approximately how long does it take the penny to hit the ground

Here's the formula for distance of fall from rest (without air resistance):

                       Distance = (1/2) (gravity) (time)²

For this problem:

                          125 m  =  (1/2) (9.8 m/s²) (time)²

Divide each side
by  4.9 m/s² :        125/4.9  sec²  =  time²

Square root
each side:              time = √(125/4.9 sec²)

                                    =  5.05 seconds    (rounded)    

By the way . . . . . Notice that we didn't need to use the mass
of the penny anywhere in the calculations.  That's because it
doesn't matter.  If there's no air resistance, then ALL objects ...
a leaf, a hammer, a dead mosquito, a rock, a feather, a car,
an ice cube, a school bus, a tree, a battle ship, a bullet shot,
a bullet dropped, a bowling ball, a tissue ... all fall at the same
rate and hit the ground at the same time.