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There is about 12 million people in the UK.About 1200 get measles every year.99% of people have spots.

1200/100=12 That is how many people that have measles with no spots every year. So 1188 is how many people have spots and measles.Roughly 10% of the 12 million children suffer from spots, which is 1.2 million.

12000000 - 1188 = 1,198,812. That is how many people have spots but no measles. So it is 1188/1198812 which is 297/299703 that is the probability of Ben having measles if he was in the uk.

If Ben however was not in the UK the probability would change depending on the population, how much people get measles every year and how much people get spots every year.