Challenge 3: Design a $bucket Age Histogram — Possible Solution
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db.users.aggregate([
  { $bucket: {
      groupBy: "$age",
      boundaries: [18, 26, 36, 51],
      default: "50+",
      output: {
        count: { $sum: 1 }
      }
  } }
])

WHY THIS WORKS AS AN ANSWER
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$bucket boundaries are [inclusive, exclusive) — each boundary value
belongs to the bucket that STARTS at it, not the one that ends there.
So boundaries: [18, 26, 36, 51] creates three buckets:

  - _id: 18  ->  ages 18 up to (not including) 26   -> "18-25"
  - _id: 26  ->  ages 26 up to (not including) 36    -> "26-35"
  - _id: 36  ->  ages 36 up to (not including) 51    -> "36-50"

That's why the boundary after 36 is 51, not 50 — a user who is exactly
50 needs to still fall inside the 36-50 bucket, and since 51 is the
EXCLUSIVE upper edge, age 50 is included while age 51 is not.

default: "50+" catches every age that doesn't fall inside any of those
three ranges — in this design, anyone 51 or older (and, as a boundary
condition, MongoDB would also reject/misplace ages below the first
boundary of 18 into default, so this design assumes no users under 18
exist in the collection).

output: { count: { $sum: 1 } } counts how many users landed in each
bucket, the same accumulator pattern as $group.