A Jogging Race
Steve, Mark and Paul jog at an average speed of 3.5, 4 and 5 km/h, respectively.
They walk on circular tracks which are 1/5, 1/4 and 3/8 of a km in circumference (in the same order) with their centres on the same straight line.
Steve, Mark and Paul start from points on this line at the same instant.
How long will it take before all three boys are again back on the line at the same time?