It's not all prime numbers odd. You forget 2 which is prime and even.
Then if p=2 and q=3 then p+q=2+3=5=r and p,q,r are all primes.
Same if p=2 and q=5 then r=7.
Check if there are any more...
Three numbers p, q and r are all prime numbers less than 50 with the property that . How many values of r are possible?
I would think this is 0 because prime numbers have to be odd and odd + odd always equals an even number. But the answer is not 0