if last character is 1 or 2 or 3 or 4 or 5 make it 0
else if ( c [ n - 1 ] == '1' c [ n - 1 ] == '2' c [ n - 1 ] == '3' c [ n - 1 ] == '4' c [ n - 1 ] == '5' ) { [ n - 1 ] = '0' ; return new string ( c ) ; } else { [ n - 1 ] = '0' ;