So the question is as follows:
A building has $s$ floors each of height $h$. An explosion destroys the strength of the $n^{th}$ floor. You may assume that the building remains intact above the floor that explodes and that the momentum is conserved in the collisions of the successive stories during the fall of the building. Estimate the time before the $(n+1)^{th}$ floor hits the ground falling vertically.
So I used the formula $$s = ut + \frac{1}{2}at^2$$
$$nh = 0(t) + \frac{1}{2}9.8t^2$$
$$t = \sqrt{\frac{nh}{4.9}}$$
But I dont know how the momentum comes into play