Ok, so with present value, we're trying to find the single lump sum you need to invest NOW, right? There are two different pieces of info you can use to work this out:
Use N= M [(1.r)n - 1 / r(1+r)^n] (FORMULA A)
- if you know how much the regular contributions (M) are
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use N = A/(1+r)^n (FORMULA B)
- if you know the final value of the person's annuity
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So say you're saving for schoolies, putting away $70 a month for 2 yrs at 9% p.a. compounding monthly, giving you a final amount of $1833, which you can find using the FV formula. Imagine your mum is gonna match the amount you save
If you tell your mum "hey ma, I'm gonna save $70 a month for 2 yrs at 9% p.a. compounding monthly..." then she will use FORMULA A, coz she knows your monthly contributions.
If you tell her "I will have saved $1833..!" then she'll use FORMULA B.
Hope that made sense somehow.