Exercise
Let p and q be distinct primes.
What is the value of 
(pq)?
p + q - 1
pq - 1
pq - p - q + 1
No, for instance
(6) = 2 
2 + 3 - 1.
(6) = 2 2 + 3 - 1.
No, elements are relatively prime to pq if they are neither divisible byp nor by q, and so there are less than pq - 1.
Yes.