Trolley A (2.0 kg) moves at 1.5 m/s towards trolley B (0.5 kg)…

Step 1. (a) $2.0\ \text{kg} \times 1.5\ \text{m/s} = 2.5\ \text{kg}\,v$, so $v = 1.2\ \text{m/s}$.

Step 2. (b) $2.0\ \text{kg} \times 1.5\ \text{m/s} + 0.5\ \text{kg} \times (-2.0\ \text{m/s}) = 2.5\ \text{kg}\,v$, so $v = 0.8\ \text{m/s}$.

Step 3. (c) (a) $2.25\ \text{J}$ before, $1.8\ \text{J}$ after. (b) $3.25\ \text{J}$ before, $0.8\ \text{J}$ after.