Solution: Find the LCM of 12 and 18. Prime factors: $12 = 2^2 imes 3$, $18 = 2 imes 3^2$. LCM is $2^2 imes 3^2 = 36$. The smallest three-digit multiple of 36 is $36 imes 3 = 108$. The population threshold is $oxed108$. - Groen Casting