Using the sinking fund formula and the information given, we have:
[tex]\begin{gathered} P=\frac{A}{(\frac{(1+\frac{r}{n})^{tn}-1}{\frac{r}{n}})}\text{ } \\ A\colon\text{ Money accumulated( in this case \$480,000) } \\ \text{ P: Periodic contribution.} \\ r\colon\text{ Interest rate ( In this case 0.05)} \\ t\colon\text{ Number of years. (In this case 21 years)} \\ n\colon\text{ Number of payments per year (in this case 12)} \\ \text{ Replacing the values, we have: } \\ P=\frac{480,000}{(\frac{(1+\frac{0.05}{12})^{21\cdot12}-1}{\frac{0.05}{12}})}\text{ } \\ P=\frac{480,000}{(\frac{(1+\frac{0.05}{12})^{252}-1}{\frac{0.05}{12}})}\text{ (Multiplying 21 by 12)} \\ P=\frac{480,000}{(\frac{(1+0.0042)^{252}-1}{0.0042})}\text{ (Dividing)} \\ P=\frac{480,000}{(\frac{2.85-1}{0.042})}\text{ (Adding and raising to the power of 452)} \\ P=\frac{480,000}{(440.81)}\text{ (Subtracting and dividing)} \\ P=1080.25\text{ (Dividing)} \\ \text{The answer is \$1080.25} \end{gathered}[/tex]