A 20 kg carton of books is at rest on a tiled floor…

Step 1. (a) $\dfrac{80\ \text{N} - 50\ \text{N}}{20\ \text{kg}} = 1.5\ \text{m/s}^2$.

Step 2. (b) $v = 1.5\ \text{m/s}^2 \times 4\ \text{s} = 6\ \text{m/s}$; $s = \tfrac12 \times 1.5\ \text{m/s}^2 \times (4\ \text{s})^2 = 12\ \text{m}$.

Step 3. (c) Slowing at $2.5\ \text{m/s}^2$: $\dfrac{(6\ \text{m/s})^2}{2 \times 2.5\ \text{m/s}^2} = 7.2\ \text{m}$.

Step 4. (d) $12\ \text{m} + 7.2\ \text{m} = 19.2\ \text{m}$.