Here's how I did it
X is the frequency a parent will be a carrier of the disease. 1/4 is the probability the parents will have a child with the disease. Therefore
X * X * 1/4 = 1/900
X = 1/15
Here's how I did it
X is the frequency a parent will be a carrier of the disease. 1/4 is the probability the parents will have a child with the disease. Therefore
X * X * 1/4 = 1/900
X = 1/15