Calculate the Rf values for the two spots on the following chromatogram.
A chromatogram diagram showing a rectangular paper strip. A solid horizontal line at the bottom is labeled 'origin'. A dashed horizontal line at the top is labeled 'solvent front'. A vertical double-headed arrow between the origin and solvent front is labeled '12.0 cm'. A spot labeled 'spot 1' is located 9.0 cm above the origin, indicated by a vertical double-headed arrow. A spot labeled 'spot 2' is located 6.0 cm above the origin, also indicated by a vertical double-headed arrow.