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Financial: Savings plan calculation
05-16-2018, 09:11 AM
Post: #1
Financial: Savings plan calculation
Dear all,

I struggle on an approach to solve the following financial calulation:

Given is a saveings plan with a fixed time t[yr], interest rate, static monthly investment and - here comes my issue - an annual bonus which depends on the annual invested money (and so is constant during the whole contract).

I'm now looking on a solution how to calculate the needed interestrate to achive the desired final capital at the end of the contract.
Without this "annual bonus" it is a no-brainer, but with I'm kind of stuck.

Any hints for me?

Regards

Carsten.
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05-16-2018, 08:02 PM (This post was last modified: 05-17-2018 04:36 AM by cortopar.)
Post: #2
RE: Financial: Savings plan calculation
Pseudo Pascal-ish code for the brute force method...
Code:

EXPORT FindInterest(years,monthly,bonus,goal);

LOCAL interest:=0.001;
LOCAL increment:=0.002;
LOCAL i:=0;
LOCAL month:=0;
LOCAL balance:=0;

WHILE balance<goal DO
  balance:=0;
  FOR i:=1 to years DO
    FOR month:=1 to 12 DO
      balance:=balance+(balance * (interest / 12))+monthly;
    END;
  
    balance:=balance+bonus;
  END;

  interest:=interest+increment;
END;

RETURN interest;
END;

Now this is going to return the interest at which balance exceeds goal. To get it exact would need additional processing at finer precision.

I’m sure there’s a formulaic way to solve this easier, but it exceeds my math abilities.

Edited to Prime-compliant HP-PPL. Tested on my Prime. It is slow as you’d expect. It could be sped up by providing or calculating a starting guess and larger increment before refining.
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Today, 04:49 PM
Post: #3
RE: Financial: Savings plan calculation
Sounds tailor-made for the spreadsheet.
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