You drop a stone into a well and hear the splash after 1.5 s…

Without air, everything falls with the same acceleration $g$ ($9.8\ \text{m/s}^2$, or $10\ \text{m/s}^2$ when the problem says so). Thrown up at $u$: it stops at the top after $\dfrac{u}{g}$, at a height $\dfrac{u^2}{2g}$, and takes as long to come down.