IMHO, it really depends on the responsibilities and what the calculation methods do. You should keep in mind the SRP (http://en.wikipedia.org/wiki/Single_responsibility_principle) to do you OO design. Also high cohesion and loose coupling principles.
If computeA(), B and C are not related, then your Adapter class should be split. If they are related, then your Adapter class should be named differently to express what it really does. For instance: TaxCalculator.
Then you could create a new class receiving a TaxCalculator and a MyValueObject which does the apply. Or just leave the apply in the calculator (though it would be two responsibilities for one class, which is usually bad). In the latter case, perhaps you could call the class TaxCalculationsSetter or something similar, and make the compute methods private. Et voila, you would satisfy again the SRP. ;-)
Always keep in mind that "what a responsibility" is, is a somewhat subjective thing.
I hope this has clarified something.