In a certain code, '253' means 'books are old';
'546' means 'man is old' and
'378' means 'buy good books'.
What stands for 'are' in that code ?
In the first and second statements, the commond code digit is '5' and the common word is 'old'.
So, '5' means 'old'.
In the first and third statements, the common code digit is '3' and the common word is 'books'.
So, '3' means 'books'.
Thus, in the first statements, '2' means 'are'.