Chapter 9 - Section 9.3 - Geometric Sequences; Geometric Series - 9.3 Assess Your Understanding - Page 665: 86

${{\$}} 6655.58$

Decreasing x by $ 15\%$ is multiplying it with $1-0.15=0.85.$ $a_{1}=15,000$ $a_{2}=15,000(0.85)$ $a_{3}=15,000(0.85)^{2}...$ This is a geometric series with $a_{1}=15,000$, $r=0.85,\ \quad a_{n}=a_{1}r^{n-1}$ We want $a_{5}=15,000(0.85)^{4}\approx 6655.58$