Primary six maths problem- concept of speed

Two horses are going to ride along a circular racecourse with circumference 3 km. The white horse rides at a speed of 32 km/h and the black one rides at 38 km/h. The start off at a point and begin riding at the same direction. When will they meet again?

(This is a primary six maths problem about the relationship between speed/ distance/ time. Please make the explanation as simple as you can. Thanks!)