A ball is thrown straight up at 18 m/s. Take g = 10 m/s². Find the greatest height it reaches.

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.