Dear Julia,
Joan will get back to you on the specifics, she is away on business right now. As far as letting your son see the solutions, that is exactly what she had in mind for this level of learning. He needs to try to figure out where he made his mistake and try to correct it himself. Perhaps it is wrong, but then he should justify his thinking sequentially and send an email to Joan directly. She asks for that kind of input and proving his work step by step will be very helpful for him in higher level math.
Nancy