[tex]a_1=15;\ r=\dfrac{1}{3}\\\\a_n=a_1r^{n-1}\to a_n=15\cdot\left(\dfrac{1}{3}\right)^{n-1}\\\\a_4=15\cdot\left(\dfrac{1}{3}\right)^{4-1}=15\cdot\left(\dfrac{1}{3}\right)^3=15\cdot\dfrac{1}{27}=\dfrac{15}{27}=\dfrac{5}{9}\\\\\\Answer:\boxed{a_4=\frac{5}{9}}[/tex]
