I have this economics problem.
What is the present worth of $500.00$ Rupiah deposited at the end of every three months for $6$ years if the interest rate is $12\%$ compounded semiannually?
According to the solution, the interest rate per quarter must be determined first. And so the solution goes by saying that:
$$(1 + i)^4 - 1 = \left(1 + \frac{0.12}{2}\right)^2 - 1$$
$i=2.96\%$ per quarter
My question is that where did the above equation came from? How was it derived?