Answer: 750
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Step-by-step explanation:
6 in the sample of 50 are infected, so 6/50 = 12/100 = 12% are infected in the sample. We expect this percentage to carry over to the entire population assuming we picked a representative random sample that isn't biased.
12% of 6250 = 0.12*650 = 750 is the expected number of infected individuals in the population.
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Here's an alternative way you can solve through a proportion
(number infected)/(total) = (number infected)/(total)
6/50 = x/6250
6*6250 = 50x ... cross multiply
37500 = 50x
50x = 37500
x = 37500/50 ... divide both sides by 50
x = 750