Tricky question please help!!!?

To celebrate graduation, you and 19 of your best friends (10 men and 10 women) charter a plane to travel around the world. Unfortunately, the plane crash lands (everyone is ok) on a deserted island. You are not found so you and your friends start a new population completely isolated from the rest of the world. Two friends have (i.e. are heterozygous for) the recessive cystic fibrosis allele (c). If as the population grows, the frequency of this allele does not change, what will be the occurance of the disease on your island?