13) 68 pence

23p (there is a stamp worth 23p)
32p (there is a stamp worth 32p)
37p (there is a stamp worth 37p)
49p (there is a stamp worth 49p)
50p (there is a stamp worth 49 p and another worth 1p. So 49 + 1 = 50)
68p (we cannot make 68p with the prices of stamps available)

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