Look at the figure for clarity. Let final position of the elevator be the dotted part.

Since the coin is undergoing free fall , the displacement in 1 sec is 4.9m taking g=9.8m/s^2

Displacement of the lift in 1 sec = a/2

and since the coin is initially at 1.8m above the floor the net displacement of the coin is also

1.8+a/2

So

1.8+a/2 = 4.9