We know that the piano loses 20% of its value each year, this means $4,000 is 80% of %5,000, $3,200 is 80% of $4,000.
For year 3, we have
[tex]0.80\cdot3200=2560[/tex]For year 4, we have
[tex]0.80\cdot2560=2048[/tex]Let's find the value after 16 years. We know that the ratio of the sequence is 0.80 and the first term is 5,000, let's use the formula for geometric sequences.
[tex]\begin{gathered} a_n=a_1\cdot r^{n-1} \\ a_{16}=5000\cdot(0.80)^{16-1}=5000\cdot(0.80)^{15} \\ a_{16}\approx175.92 \end{gathered}[/tex]