This could mean 15m represents 15×600=9000m, so in scale 1:300 it represents 15×300=4500m.
Or it could mean 15m is represented by 15/600=1/40 metre=25mm or 2.5cm.
So 15m is represented by 15/300=1/20 metre=50mm or 5cm. (I think this answer is probably the expected one. In this 1cm represents 300cm=3m, so 5cm represents 15m, as might be applied in a building plan.)
It depends on whether 15m is represented or is to be represented by. The question doesn't clarify this.