Answer: 12,500,000 .
Explanation:
Given : The fraction of people over 65 years in United Kingdom is affected by Alzheimer =

Also, the number of affected people over 65 years= 250,000
According to the question :
(Number of affected people) =( fraction of people affected) x (Total people)
Then, (Total people) = (Number of affected people) ÷( fraction of people affected)

Hence, the number of people over 65 are 12,500,000 .