A stone is dropped from the top of a building $45$ m high. How long does it take to reach the ground? (Take $g = 10$ m s$^{-2}$.)

From $h = \dfrac{1}{2}gt^2$: $45 = 5t^2$, so $t^2 = 9$ and $t = 3$ s. Dividing height by $g$ gives $4.5$, which has the wrong units and ignores the square.