Question

If I have $500 in my account after 4 years investing at 2.5% per year, how much money did I start with?

Answer 1

using the same form as above:
a*1.025^t = 500.
a = 500/(1.025^t), where t=6 and a is the starting amount.
Thus:
a = 500/(1.025^6) = 431.148433 = 431.15$