If,
29 - 1 = 30
9 - 1 = 10
14 - 1 = 15

Based on similar logic, Can you prove that the below algebraic equation is true?
11 - 1 = 10 ?

XI - I = X

Remove I (Roman equivalent of number 1) from the LHS, you will the get the number on the RHS.
29 - 1 = 30
XXIX - I = XXX

14 - 1 = 15
XIV - I = XV

9 - 1 =10
IX - I = X

Similarly,
11 - 1 = 10
XI - I = X

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