If 8 grapefruits, 7 oranges and 3 lemons weigh as 3 oranges, 6 grapefruits and 6 lemons, and if a grapefruit weighs  2/3 of a lemon , while dozen of oranges 3 chilos...
Which is the weight of 1 lemon?

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Where? Just a clue: the quantity of fish in a recipe which has as ingredient a twig of thyme.