Interviewer view · keep this screen to yourself

Lifting a London sandwich line's output

You run the case. Read the prompt, answer questions from the notes below, and share data only when the candidate asks for it or gets stuck. Score at the end.

Case timer

00:00

Target length:

1. Read the prompt aloud

Read it slowly, then pause. Let the candidate ask questions before they structure.

A sandwich kitchen in London has the step capacities in the chart above. Lunch demand is 85 sandwiches an hour over a three-hour peak, and each sandwich earns GBP 3 of contribution. A second assembly worker would raise Assemble to 100 an hour and cost GBP 30,000 a year. Is it worth it?

The prompt refers to Exhibit 1. After reading it, say: "Open Exhibit 1 now."

Format note: Interviewer-led: the interviewer shows the capacity chart and asks for the bottleneck, the value of fixing it, and what limits the line next.

2. Answers to clarifying questions

Give these answers only if the candidate asks. If they ask something not listed, give a sensible answer or say it does not matter here.

If asked: Is demand higher than what we make today?
Answer: Yes, about 85 sandwiches an hour at lunch; customers leave when the queue is long.
If asked: How long is the peak, and how many days a year?
Answer: About three hours a day, on about 250 working days.

3. The hypothesis a strong candidate states

Listen for an early, testable guess like this one. It does not need to match word for word.

Assemble is the slowest step, so my hypothesis is that it caps output below demand, and that adding capacity there, and only there, pays for itself.

4. A model structure

Compare the candidate's structure with this one. A different split can be just as good if it is clean and fits the problem.

Output equals the lower of demand and the slowest step
- Find the bottleneck and compare with demand
- Key: Value of the lost sales
- Cost of the fix and the next constraint

Exhibit 1

The prompt uses this exhibit, so the candidate opens it right after you read the prompt ("Show exhibit 1" on their screen).

Steps and capacity on a sandwich line(sandwiches per hour)

So-what

The whole line can make only 60 an hour because Assemble is the bottleneck. Speeding up Prep or Wrap changes nothing until Assemble improves.

5. The working, step by step

Each step shows how a strong candidate works it out. Share a new fact from it only when the candidate asks or is stuck, and let them do the math: the result in the dark box is what they should reach.

Step 1: Find the bottleneck
What a strong candidate does: The slowest step is Assemble at 60 an hour, so the line makes 60 an hour, below lunch demand of 85.
Slowest step (sandwiches per hour): min(120; 60; 100; 90) = 60
Step 2: Lost sales per hour
What a strong candidate does: Demand the line cannot serve at peak.
Lost sandwiches per peak hour: 85 - 60 = 25
Step 3: After the fix
What a strong candidate does: With Assemble at 100, the slowest step becomes Checkout at 90, which is above demand of 85. The line can now serve all lunch demand.
Slowest step after the fix (sandwiches per hour): min(120; 100; 100; 90) = 90
Step 4: Value of the fix
What a strong candidate does: 25 extra sandwiches an hour at GBP 3 each, for 3 peak hours on 250 days.
Extra contribution (GBP a year): 25 × 3 × 3 × 250 = 56,250
Step 5: Net gain
What a strong candidate does: Minus the GBP 30,000 cost of the second worker.
Net gain (GBP a year): 56,250 - 30,000 = 26,250

The recommendation to listen for

At the end, say: "The CEO walks in. What is your recommendation?"

Add the second assembly worker. First, Assemble caps output at 60 an hour, so about 25 sandwiches an hour of lunch demand are lost. Second, the fix serves all 85 an hour and adds about GBP 56,250 of contribution a year for GBP 30,000 of cost, a net gain of about GBP 26,250. Third, spending on Prep or Wrap would change nothing, because they already have spare capacity. Checkout, at 90 an hour, becomes the next limit if lunch demand grows above 90.

Risks a strong answer names: Lunch demand may be lower on some days, reducing the gain; Two assemblers may get in each other's way in a small kitchen.

Next steps: Trial the second assembler for four weeks and measure output at peak.

Strong versus weak

A strong answer

Checked demand, found the bottleneck, valued the lost sales, compared with the cost of the fix, and named the next constraint.

A weak answer

Suggested faster prep equipment, which was never the limit, and never put a money value on anything.

Score the candidate

Score each criterion from 1 to 5. A 2 or a 4 sits between the descriptions.

Structuring

1: No clear structure, or a memorized framework that does not fit the problem.
3: A sensible structure, but with some overlap, a gap, or no clear priority.
5: A structure built for this problem, with no overlap and no gaps, a stated hypothesis, and a clear first step.

Math

1: Frequent errors, no units, or needed heavy help to finish.
3: Right answers but slow or messy working, or one error caught late.
5: Accurate and clearly narrated, with units, a sense check, and what the number means.

Exhibit reading

1: Read the chart aloud without finding the number that matters.
3: Found the key number but gave only part of the so-what.
5: Pulled the right number quickly and said what it means for the case.

Synthesis

1: Summarized what was done, with no clear answer.
3: Gave an answer, but with weak reasons or no next step.
5: Answer first, then two or three reasons with numbers, the main risk, and next steps.

Communication

1: Hard to follow: long silences, no signposting.
3: Clear most of the time, with some rambling.
5: Calm and concise, signposted each step, and checked in with the interviewer.

Total

0 out of 25

Score all five criteria to see the band and the feedback template.