To find the cost of one marker, you need to divide the total cost of the markers by the number of markers in the set.
In this case, Lainey bought a set of 20 markers for $6.
Cost of 1 marker = Total cost / Number of markers
Cost of 1 marker = $6 / 20 markers
Cost of 1 marker = $0.30
Therefore, the cost of one marker is $0.30.