The activity of a radio active sample decreases to one third of its original value in 9 years, what will be the activity after further 9 years?

After every 9years the value becomes one third of the initial value

So after a total of 18yrs the value will become one ninth of the initial value

${\mathrm{A}}_{18\mathrm{yrs}}={\left(\frac{1}{3}\right)}^2{\mathrm{A}}_{\mathrm{o}}=\frac{{\mathrm{A}}_{\mathrm{o}}}{9}$