A 45 kg girl and a 60 kg boy stand at rest on smooth ice and push each other away…

Step 1. (a) $45\ \text{kg} \times 2.4\ \text{m/s} = 60\ \text{kg}\,v$, so $v = 1.8\ \text{m/s}$, the other way.

Step 2. (b) In $4.0\ \text{s}$: $9.6\ \text{m}$ and $7.2\ \text{m}$, so $16.8\ \text{m}$ apart.

Step 3. (c) Still where they started: no external horizontal force acts.