The formula is M = P × n + (P × n(n+1)r) / (2400), beta. They use n(n+1)/2 because the first deposit earns interest for 'n' months, the next for 'n-1', and so on—it’s an arithmetic progression, and that sum simplifies to that neat expression. I explained this to my son just last week, the clatter of my typewriter keeping rhythm with the calculation, and it reminded me how even small, regular savings grow with patient discipline, hai na?
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