Find the number of the positive integers with three digits which can be written using
the digits \(2\),
\(3\),
\(4\) and
\(5\). Each
digit can be used at most once.
Find the value of the real parameter
\(a\) which ensures, that
the following two lines \(p\)
and \(q\)
are perpendicular.
\[
p\colon ax + y - 4 = 0,\qquad q\colon x + 2y + 4 = 0.
\]