Concept:Equal ratios property: The common ratio equals sum of numerators divided by sum of denominators.Explanation:Let the common value be k.So, k=q+rp​=r+sq​=s+pr​=p+qs​.Sum of numerators: p+q+r+s.Sum of denominators: (q+r)+(r+s)+(s+p)+(p+q)=2(p+q+r+s).Since all p,q,r,s>0, denominators are positive.Thus, k=2(p+q+r+s)p+q+r+s​=21​.Answer:21​