The answer is 150,000,000 years (150 million years)
Let's first convert 7,500 km to cm: 1 km = 100,000 cm 7,500 km = 7,500 · 1 km = 7,500 · 100,000 cm = 750,000,000 cm
Now, it is known that the plate is spreading at 5 cm per 1 year. Make a proportion: 5 cm : 1 year = 750,000,000 cm : x x = 750,000,000 cm · 1 year ÷ 5 cm = 150,000,000 years
Father is dd Mother is Dd (has to be because one child has the disease)
Parents cross is dd x Dd which gives rise to Dd (0.5) and dd (0.5). Each time they have a child they have a 50% chance of the child having the disease. In their case, only one of their 3 children is dd. The others though are carriers!
