Two boats, travelling at 5 km/h and 10 km/h, head directly towards each other. They begin at a distance of 20 km from each other. How far apart are they (in km) one minute before they collide?

Answer: Option C

In the final minute of collision, the two boats travel $$\frac{5}{60}$$ km and $$\frac{10}{60}$$ km.
As they moves towards each other relative distance covered in 1 minute,
$$\begin{array}{rl}& =\frac{1}{12}+\frac{1}{6}\\ & =\frac{1}{4}\text{km}\end{array}$$