$8.($a$)$ Suppose Studebaker's goal is to accumulate $$400,000$ in $20$ years. Assume the $$30,000$ is invested at $8$ percent. How much will he have to save in equal amounts at the end of each of the next $20$ years if he can earn $8$ percent per year on any investments?

$($b$)$ Repeat part $($a$)$ but assume he will not be able to save any money in years $13$ to $20.$ This is$,$ he will save an equal amount at the end of years $1$ to $12$ and nothing thereafter.

Can someone please explain how to do this in really simple terms? Especially how to determine the equal yearly amount needed.