They are giving you a ratio between the two quantities. So if x households used both brands of soap, this is the overlap region in the typical two category Venn diagram, then the number of households that use only brand B soap is 3x. The region of households that use brand B soap only is the region left if you took out the households that use both brand A and brand B soap. It is best to visualize this by categorizing the regions in Venn diagram.

Ashna says

2017 OG

Diagonistic PS

Q:6

Can you please explain the line, ‘for every household that used both brands of soap, 3 used only brand B soap’

GMAT Quantum says

They are giving you a ratio between the two quantities. So if x households used both brands of soap, this is the overlap region in the typical two category Venn diagram, then the number of households that use only brand B soap is 3x. The region of households that use brand B soap only is the region left if you took out the households that use both brand A and brand B soap. It is best to visualize this by categorizing the regions in Venn diagram.