[tex]\text{P.G.} = (20, 10, 5, ...) \\\\ \bullet a_1 = 20 \\\\ \bullet q = \frac{10}{20} = \frac{1}{2} \\\\ \bullet a_n = a_1 \cdot q^{n - 1} \\\\ a_{10} = 20 \cdot (\frac{1}{2})^{10 - 1} \\\\ a_{10} = 20 \cdot (\frac{1}{2})^{9} \\\\ a_{10} = 20 \cdot \frac{1}{512} \\\\ a_{10} = \frac{20}{512} = \boxed{\frac{10}{256}}[/tex]
