Build the Service Layer

public class AccountHealthService {
    public Integer getOpenCaseCount(Id accountId) {
        return [SELECT COUNT() FROM Case WHERE AccountId = :accountId AND IsClosed = false];
    }

    public Decimal getAverageResolutionDays(Id accountId) {
        List<Case> closedCases = [
            SELECT CreatedDate, ClosedDate FROM Case
            WHERE AccountId = :accountId AND IsClosed = true AND ClosedDate != null
        ];

        if (closedCases.isEmpty()) {
            return 0;
        }

        Integer totalDays = 0;
        for (Case c : closedCases) {
            totalDays += c.CreatedDate.date().daysBetween(c.ClosedDate.date());
        }
        return totalDays / (Decimal) closedCases.size();
    }
}

1. public class AccountHealthService { — the class.
2. public Integer getOpenCaseCount(Id accountId) { — the first metric method.
3. return [SELECT COUNT() FROM Case WHERE AccountId = :accountId AND IsClosed = false]; — Module 20's COUNT() aggregate function, filtered to this Account's still-open Cases.
4. } — closes getOpenCaseCount.
5. (blank line)
6. public Decimal getAverageResolutionDays(Id accountId) { — the second metric method.
7. List<Case> closedCases = [ — opens a SOQL query, since averaging a date difference needs the actual records, not just a count.
8. SELECT CreatedDate, ClosedDate FROM Case — only the two fields the average needs.
9. WHERE AccountId = :accountId AND IsClosed = true AND ClosedDate != null — closed Cases for this Account that actually have a resolution date.
10. ]; — closes the query, assigning the results to closedCases.
11. (blank line)
12. if (closedCases.isEmpty()) { — Module 17's "Handle Edge Cases" habit, checked up front.
13. return 0; — an Account with no closed Cases yet has no average to compute — 0 rather than a divide-by-zero error.
14. } — closes the if.
15. (blank line)
16. Integer totalDays = 0; — an accumulator, initialized before the loop.
17. for (Case c : closedCases) { — iterates the already-queried list — no SOQL inside this loop.
18. totalDays += c.CreatedDate.date().daysBetween(c.ClosedDate.date()); — Apex's Date.daysBetween gives the whole-day gap between the two dates, added to the running total.
19. } — closes the for loop.
20. return totalDays / (Decimal) closedCases.size(); — casting to Decimal first ensures true division rather than truncating integer division.
21. } — closes getAverageResolutionDays.
22. } — closes the class.

getOpenCaseCount uses Module 20's COUNT() directly; getAverageResolutionDays computes the average in Apex since it needs a date difference SOQL's AVG() alone can't express — both handle the empty-data case (Module 17's "Handle Edge Cases" habit) up front.

public class AccountHealthSummary {
    public Integer openCaseCount;
    public Decimal averageResolutionDays;
    public Decimal totalWonRevenue;
    public Integer daysSinceLastActivity;
}

public AccountHealthSummary getHealthSummary(Id accountId) {
    AccountHealthSummary summary = new AccountHealthSummary();
    summary.openCaseCount = getOpenCaseCount(accountId);
    summary.averageResolutionDays = getAverageResolutionDays(accountId);
    summary.totalWonRevenue = getTotalWonRevenue(accountId);
    summary.daysSinceLastActivity = getDaysSinceLastActivity(accountId);
    return summary;
}

1. public class AccountHealthSummary { — a plain wrapper class, no annotations at all yet.
2. public Integer openCaseCount; — one field per metric.
3. public Decimal averageResolutionDays; — same.
4. public Decimal totalWonRevenue; — same.
5. public Integer daysSinceLastActivity; — same.
6. } — closes the wrapper class.
7. (blank line)
8. public AccountHealthSummary getHealthSummary(Id accountId) { — the combining method.
9. AccountHealthSummary summary = new AccountHealthSummary(); — one instance to populate.
10. summary.openCaseCount = getOpenCaseCount(accountId); — delegates to the focused method already written.
11. summary.averageResolutionDays = getAverageResolutionDays(accountId); — same.
12. summary.totalWonRevenue = getTotalWonRevenue(accountId); — same (implemented in this lesson's exercise).
13. summary.daysSinceLastActivity = getDaysSinceLastActivity(accountId); — same.
14. return summary; — one populated object, ready for a caller.
15. } — closes getHealthSummary.

This is Module 34's DTO/wrapper-class pattern, used here at the service layer rather than directly in the controller — AccountHealthSummary doesn't yet have any @AuraEnabled annotations, since this class has no idea an LWC will ever use it (that's the controller's job, in a later lesson).

AccountHealthService and AccountHealthSummary could be called from an LWC controller, a batch job, or a plain Execute Anonymous script equally easily — nothing here mentions @AuraEnabled, cacheable, or anything LWC-specific. This mirrors Module 26's AccountRollupService: the logic that computes something and the logic that exposes it to a UI are deliberately kept as separate concerns.