How long will it take an object dropped from a window to fall a distance of 78.4 meters?

As you said it was dropped so in that case I'll take 78.4 meters as the height ...

we know that

h = ut + 0.5*g*t (sqr)

t (sqr) = 78.4 / (0.5*9.8) [g = 9.8 ms-2]

t = √16

t = 4

So it'd take 4 seconds