The speed of a 5kg object that fell from the rest of the total distance of 100 meters ( assume no air resistance ) and g=10m/s^2)

44.72m/s
In this question you are asked about the speed of the object when assuming no air resistance, then the mass (5kg) doesn't need to be used. The formula you should use in this case is:

t(time)= 
t=
t= 4.472 second

v(speed/velocity)= g (gravitation/acceleration) x t(time)
v= 10m/ x 4.472 s
v= 44.72m/s