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Hi,
Let x represent the number of hats and 1/4x the number of brown hats.
Question states selling 4/5 of the brown hats Or having 1/5 of the brown left.
(1/3)x total hats left of which 1/5*1/4x brown are left
(1/20)x / (1/3)x = 3/20, fraction of the unsold hats that were brown