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Thiago

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Considering a population "x" at the beginning of 1991 and "y" at the end, we have: y = x.(1+1/4) = x.5/4 So now we have to decrease the population y by i% to get x: x = y.(1-i) x = x.5/4 - x.5/4.i 1 - 5/4 = -5/4.i i = 1/5 So y has to be decreased by 20%.
So, any non-prime number is formed by a product of prime numbers, then if 9991 isn´t prime, we can find its factors. Given that 9991 is a four-digit number, we can start dividing it by primes that are at least bigger than 90 (given that 90^2= 8100), so

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