Sum salary by department, only if salary > 70k .Where(e => e.Salary > 70000) .GroupBy(e => e.Department)?

Short answer: var sum = employees .Select(g => new { g.Key, Sum = g.Sum(e => e.Salary) });

Example code

var sum = employees
.Select(g => new { g.Key, Sum = g.Sum(e => e.Salary) });

Real-world example (ShopNest)

ShopNest filters active products with products.Where(p => p.IsActive && p.Stock > 0) before showing the catalog.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.