RGS's solution for Problem 5, Phase 2 of 2020 APL competition (see https://mathspp.com/blog/2020-apl-competition for my thoughts on it)

P5Phase2_2020.dyalog

⍝ PROBLEM 5.1 
rr ← {
    (⎕IO ⎕ML ⎕WX) ← 0 1 3
    ⍝ Computes how much your investment is worth after cash flows and rate calculations.
    ⍝ Dyadic function expecting number vector on the left (deposits/withdrawals) and number vector on the right (corresponding rates).
    ⍝ At moment 1 you have r[1] ← ⍺[0] and at the subsequent moments you have r[n] ← ⍺[n-1] + r[n-1]×⍵[n-1].
    ⍝ Expanding the recurrence relation we can determine beforehand all expressions.
    ⍝ Assuming ⎕IO←0 and traditional mathematical notation,
    ⍝ r[1]←⍺[0], r[2]←⍺[1]+⍺[0]⍵[1], r[3]←⍺[2]+⍺[1]⍵[2]+⍺[0]⍵[2]⍵[1], r[4]←⍺[3]+⍺[2]⍵[3]+⍺[1]⍵[3]⍵[2]+⍺[0]⍵[3]⍵[2]⍵[1]
    ⍝ Returns a number.

    ⍝ This code manipulates triangular matrices as seen in https://aplcart.info/?q=extract%20triangular%20matrix#,
    ⍝  content I added to APL Cart, cf. https://github.com/abrudz/aplcart/pull/46
    _E ← {(⊢×∘.⍺⍺⍨∘⍳∘≢)⍵}  ⍝ Extract the lower/upper (strict?) triangular part of a matrix
    _M ← {(∘.⍺⍺⍨)⍵}        ⍝ Vector into rows (⊢) or columns (⊣) of a square matrix
    rt ← ×⍀ 1 + >_E ⊣_M ⍵  ⍝ Produce cumulative rates,
    a ← ≥_E ⊢_M ⍺          ⍝ expand amounts to proper shape
    +/ a × rt              ⍝ and sum them after multiplying.
}

⍝ PROBLEM 5.2                                                
⍝ Computes the present value of an investment with future cash flows ⍺ at inflation rates ⍵.
⍝ Dyadic function expecting two vectors with the same length.
⍝ Returns a number.
pv ← (1⊥×∘(×\1÷+∘1))