Mysterious Cryptogram

You received a new cryptogram from Graham. The cryptogram is [ 7 17 26 10 / 11 14 23 10 / 20 19 / 25 26 23 25 17 10 / 14 24 17 6 19 9 / 25 13 14 23 9 / 11 17 20 20 23 ] [Key : 5]

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