If you pay the Durham poll tax, then you are 18 years or older. If you are 18 years or older, then you pay the Durham poll tax.

The required biconditional statement is: You pay the Durham poll tax if and only if you are 18 or old.

Step-by-step explanation:

In the provided statement,

The statement is: If you pay the Durham poll tax, then you are 18 years or older.

The converse is: If you are 18 years or older, then you pay the Durham poll tax.

The combination of a conditional statement and its converse is called biconditional statement.

The biconditional statement contains if and only if phrase between two part of the statement.

Which means the statement and converse both are true.

Therefore, the required biconditional statement is: You pay the Durham poll tax if and only if you are 18 or old.