Just take an example and see:
Take some easy numbers 2,4,6
The average of 2&4 is 3
The average of 4&6 is 5
The average of 2&6 is 4
5 > 4 > 3
So we let P=5, Q=4, R=3, so that P > Q > R
That means:
average of 4&6 > average of 2&6 > average of 2&4
So we have to let a=4, b=6, c=2
And since 6 > 4 > 2, that means b > a > c, choice D
Edwin