We need to multiply to get the population of North Folk.
[tex]\frac{18}{25} \times 9000 = \frac{18\times 4}{25\times 100} \times 9000 = \frac{72}{100} \times 9000\\\\\frac{72}{100} \times 9000 = 72 \times \frac{9000}{100} = 72\times 90 = \boxed{\boxed{6480}}[/tex]
6480 is your answer.
