the acceleration due to gravity on planet x is
one fifth that on the surface of the earth.
if it takes 3.9 s for an object to fall a certain
distance from rest on earth, how long would
it take to fall the same distance on planet x?
answer in units of s.

If Fg=mg=ma and, Fg(planetX)=1/5Fg(earth)

then the time would be 5x of the time as gravity is acceleration. So 3.9s*5=19.5s

As the force of gravity is less, then the acceleration of masses is also less, therefore it will take more time for the object to fall by the factor of the force of gravity difference

D= 1/2 g T^2

T = √(2D/g)

If g changes to g/5, then T increases by the factor √5 .

If it was originally 3.9 sec, then the new T is 8.72 sec.