Use a genetic diagram to work out the probability that the offspring have cystic fibrosis if both parents are carriers.

CF is a homozygous recessive condition. This means that it is inherited, ie a child gets the condition by the genetic material passed on to it from its parents. Let's call the faulty allele f (which is also the recessive allele) and let's call the non-faulty allele F (which is also the dominant allele). 
If both parents are carriers this means they have the genotype Ff. Being a carrier remember means that you still have one faulty allele but you do not present with symptoms as you do not have the disease because in order to be affected you must have 2 faulty alleles.
(In my lesson I would then draw out the genetic diagram to explain how the ratios can be calculated.)