Add q to p, add a to the result, then divide r by what you have
Add q to p:
p + q
Add a to the result:
p + q + a
Then divide r by what you have:
r/(p + q + a)
Add q to p:
p + q
Add a to the result:
p + q + a
Then divide r by what you have:
r/(p + q + a)