A 1000 kg cannon on smooth flat ground fires a 10 kg ball at 120 m/s to the right…

Step 1. (a) $10\ \text{kg} \times 120\ \text{m/s} = 1200\ \text{kg m/s}$ right.

Step 2. (b) $\dfrac{1200\ \text{kg m/s}}{1000\ \text{kg}} = 1.2\ \text{m/s}$ to the left.

Step 3. (c) $\dfrac{1200\ \text{kg m/s}}{2000\ \text{kg}} = 0.6\ \text{m/s}$: half as fast.