For a certain gas the ratio of specific heat is 1.5, for this has:
Cp=5RJ
Cv=3RJ
Cv=5RJ
Cp=3RJ
specific heat ratio = γ=CpCv =1.5 ⇒ Cp=1.5Cvdifference of specific heats is Cp-Cv=RJ substitute given values in above equation and Cp-Cp1.5=RJor Cp=RJ(1−11.5)=3RJ.