Any help on this would be wonderful

5. A cross-linked rubber sample swells to 10 times its original volume when immersed in solvent A. Assuming that this is the best possible solvent for the polymer, what is the solubility parameter of solvent B, which is made up by adding a small amount of a lactone to solvent A? The sample swells only to 5 times its original volume in B. Both solvents A and B have a molar volume of 100 cm and the same lattice parameter B. All tests are made at 27C. Solvent A: CED= 85.0 cal/cm', X=0.33