average[num_] := Apply[Plus,num]/Length[num]

dm[m1_,m2_] := 200 (m1 - m2)/(m1 + m2)

pn = average[{938.,939.}]

sigma = average[{1197.,1192.,1189.}]

cas = average[{1321.,1314}]

dm[pn,1115]

dm[pn,sigma]

dm[sigma,cas]