(a) The answer depends on which state or territory law is applied to the case. In some jurisdictions the common law still applies, but some states (NSW, Vic and SA) have enacted legislative provisions. Under the common law, performance already made before frustration (such as the delivery of goods) cannot be recovered unless nothing at all has been received in return - sometimes referred to as 'a total failure of consideration'. In this case, B received five articles, and so could not recover any of the money paid to A.
In states where there is relevant legislation, the result would be different. The Acts generally allow the parties to recover a proportionate amount of money paid if the expected counter-performance was partly frustrated, or fair compensation for performance made but not paid for before frustration of the contract.