Aggregation and the statement

Balances and totals

decimal balance = ledger.Sum(t => t.Dir == Direction.Income ? t.Amount : -t.Amount);

var byCategory = ledger
    .GroupBy(t => t.Category)
    .Select(g => new { Category = g.Key, Total = g.Sum(t => t.Amount) })
    .OrderByDescending(x => x.Total)
    .ToList();

The signed fold gives the running balance; the GroupBy pipeline is the category report. Both are one pass over data — the aggregation lesson's patterns applied to a real domain.

The statement

A monthly statement composes them: filter the month, compute income/expense/balance, find the biggest category, take the top 3 entries by amount descending. Nothing new — Module 15's Where/Sum/GroupBy/OrderByDescending in combination, which is the point: capstones verify that earlier modules became reflexes.