Answer :

35cm!

4 + 4 = 8

7 + 7 = 14

13 + 8 =  21 (13 + (8 - 1 ) = 20 + 1 = 21)

21 + 14 = 35 (4 + 1 = 5, 20 + 10 = 30, 30 + 5 = 35)

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