The preposition in the given sentence "Gracie and Helen had not seen each other for 50 years" is 'for'.
<h3>What is a preposition?</h3>
A preposition is a word or phrase that appears before a noun, pronoun, or noun phrase to indicate direction, time, place, location, spatial relationships, or to introduce an object.
Prepositions include words like "in," "at," "on," "of," "to", etc.
Thus, the preposition in the given sentence "Gracie and Helen had not seen each other for 50 years" is 'for'.
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