The letter weighted 7 ounces
Hopefully the math terms won't confuse anyone.
So in the series, a_6 = 9 and a_8 = 15. Since it's an arithmetic sequence, all the terms in the series can be represented as:
a_n = a_0 + n * k
where n is the nth number. K is a constant you add to reach the next term, and a_0 is the starting constant.
a_6 = a_0 + 6 * k
a_8 = a_0 + 8 * k
solving this system gives you that
k = 3 and a_0 = -9
for any term n for this arithmetic sequence,
a_n = -9 + 3 * k
I might be wrong, but I think it is 8.
I think it’s 8 because all side lengths are equal in a square, meaning if one side length is 8 all the other side lengths are going to be 8 also.
Compound interest with addition formula:

where,
A = final amount
P = initial principal balance
r = interest rate
n = number of times interest applied per time period
t = number of time periods elapsed
PMT = Regular contributions (additional money added to investment)
in this example
P = 2500
r = 8% = 0.08
n = 4
t = 5 years
PMT = 2500

solving for A:

Therefore, his investment after 5 years will be
$189,408.29