Derive and use the finite geometric series formula to solve problems such as mortgage-payment models.
Compare financial plans at a matched valuation date
Problem
Plans begin together. Plan A deposits \(\$100\) at each month-end for \(\text{months}~1\text{--}12\). Plan B deposits \(\$120\) at each month-end for \(\text{months}~1\text{--}10\) and makes \(\text{no}~\text{later}~\text{deposits}\). Both use monthly factor \(1.005\). Compare immediately after \(\text{month}~12\).
A fair financial comparison requires one shared valuation date. Accumulate each plan through its own last deposit, then carry any earlier-ending balance forward through the remaining periods even when no new deposits occur. Only after both balances refer to the same instant should they be subtracted and compared.
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