The robot trip is faster. Why are orders still late?
Three steps, one extra worker, and 60 orders due in the first hour.
The question
Rill Works is fictional. This warehouse starts empty. Sixty orders arrive at minutes 0 through 59. Every order visits pick, pack and check. Each step has one worker. Work continues after minute 60; there are no breaks or defects.
| Seconds | |
|---|---|
| Pick | 48 |
| Pack | 72 |
| Check | 45 |
A second worker can work in parallel at one step. Each worker handles one order at a time.
“Where should the next worker go? State hourly capacity and the number finished by minute 60.”
Not saved by this site. Your browser may restore text when you navigate back.
A strong answer, step by step
Open each step in turn. This is one strong answer, not the only one.
Step 1: Convert time into capacity
What would you do here? Open the move you’d make.
Add the three capacities.
Tempting, but it misses something. Every order visits every step; parallel capacities cannot be added.
Calculate each station’s capacity.
The strong move. Divide 3,600 seconds by the time per order.
See how a strong candidate says it
“Pick can handle 75 per hour, pack 50 and check 80.”
Why it works: Divide 3,600 seconds by the time per order.
Step 2: Find the limiting step
What would you do here? Open the move you’d make.
Add the worker at packing.
The strong move. Every order must pass through packing.
Speed up picking first.
Tempting, but it misses something. The packer can still process only 50 orders per hour.
See how a strong candidate says it
“Packing limits nominal output to 50 per hour.”
Why it works: Every order must pass through packing.
Step 3: Keep the starting state visible
What would you do here? Open the move you’d make.
Report 50 completed orders.
Tempting, but it misses something. An empty line spends part of the first hour filling its downstream steps.
Trace arrival, wait and processing time.
The strong move. Nominal capacity is not the count completed during the first hour.
See how a strong candidate says it
“The empty-start trace finishes 48 orders by minute 60; the last order finishes at 73.55 minutes.”
Why it works: Nominal capacity is not the count completed during the first hour.
Step 4: Check the new constraint
What would you do here? Open the move you’d make.
Report both capacity and completed orders.
The strong move. Picking becomes the constraint; the final orders still need time to cross the line.
Claim all 60 are finished by minute 60.
Tempting, but it misses something. Orders arriving near minute 60 still need processing time.
See how a strong candidate says it
“Two packers lift nominal capacity to 75 per hour. The first hour finishes 58 orders; the final order ends at 61.75 minutes.”
Why it works: Picking becomes the constraint; the final orders still need time to cross the line.
The answer, in short
- Decision
- Add the worker at packing.
- Because
- It is the current constraint; faster picking alone cannot remove the pack queue.
- What would change it
- At demand of 80 per hour, the expanded line still has only 75 per hour of pick capacity.
Picking now takes 36 seconds per order while packing stays unchanged. What happens to nominal output?
Common traps
- Confusing capacity with empty-start completions.
- Optimizing the fastest step.
Check your answer
- I converted seconds to hourly capacity.
- I identified packing as the constraint.
- I gave the first-hour completion count.
- I checked the next constraint.
Last reviewed Nov 10, 2026