A ball is thrown straight up at 20 m/s. Take g = 10 m/s².

Step 1. (a) $0 = 400\ \text{m}^2/\text{s}^2 - 20\ \text{m/s}^2 \times H$, so $H = 20\ \text{m}$.

Step 2. (b) $v^2 = 400\ \text{m}^2/\text{s}^2 - 2 \times 10\ \text{m/s}^2 \times 15\ \text{m} = 100\ \text{m}^2/\text{s}^2$, so $v = \pm 10\ \text{m/s}$: $10\ \text{m/s}$ up on the way up, $10\ \text{m/s}$ down on the way down.

Step 3. (c) $2\ \text{s}$ up and $2\ \text{s}$ down: $4\ \text{s}$.