Two parents are both carriers for a genetic condition which is recessive. What is the percentage chance they will have a child affected by the condition?

I would approach explaining the answer to this question in a manner which was appropriate to the stage and understanding of the student. First I would check understanding of the meaning of the question and if they were familiar with the concepts of 'recessive' and 'dominant' conditions. I would ask them how they had previously been taught to approach such questions. Personally I would recommend nomenclature "Xx" for each parent as carriers for a recessive condition with 'x' representing the recessive condition in question. I would then direct them through setting up and filling in a punnett square resulting in four possible genotypic outcomes - "XX" "Xx" "Xx" and "xx". I would highlight that "xx" denotes the affected child as in order for a recessive condition to be expressed phenotypically the individual must be homozygous for that condition genotypically.To finalise answering the question after having carried out the working for it (using diagrams if necessary) I would discuss how this may be answered and highlight that in this case it is by a percentage (1/4 = 0.25x100 = 25%) but that in other questions this may be by a ratio (1:2:1 genotypes, 3:1 phenotypes) etc and that they should not be put off by how to answer these differently formatted versions of the same question. I would check understanding before finalising and ask for any further follow up questions or meanings of terms used. Above all I would try to tailor the explanation to the student in question and if this approach did not work I would try another one to do the best job of tutoring that is possible!