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Why a Johannesburg car-service workshop runs out of time
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
1. Read the prompt aloud
Read it slowly, then pause. Let the candidate ask questions before they structure.
A chain of car-service workshops in Johannesburg is losing customers because cars are not ready on time and the workshops turn bookings away. The owner thinks each workshop needs more bays. What would you do?
Format note: Candidate-led: you map the process and ask for the numbers; the interviewer answers only what you ask.
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: What is the complaint exactly: long waits, or cars not finished the same day?
Answer: Both. Customers book a same-day service, and many cars are not ready by closing time.
If asked: Do we turn customers away?
Answer: Yes. When the day is full, callers are told to come another day, and many go to a rival.
If asked: Is this one workshop or all of them?
Answer: Start with one typical workshop; the others are similar.
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.
A workshop is limited by its service bays, so my hypothesis is that bays are the bottleneck, and that something other than the repair work itself, such as waiting for parts, is using up bay time.
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.
- Bay capacity versus demand, and what uses up bay time
- Demand: cars arriving per day
- Key: Capacity: bays x hours / hours each car holds a bay
- Repair work
- Key: Waiting for parts in the bay
- Value of recovered capacity and cost of the fix
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: Capacity in theory
What a strong candidate does: Candidate: "How many bays, how long is the day, and how long does a service take?" Interviewer: "Six bays, open nine hours, and a service is about two hours of work."
Cars per day in theory: 6 × 9 ÷ 2 = 27
Step 2: Compare with demand
What a strong candidate does: Interviewer: "About 24 cars a day want a service." Candidate: "In theory the bays should cope, at about 89 percent use. So something else is using bay time."
Required utilization in theory (%): 24 ÷ 27 × 100 = 88.89
Step 3: Find the hidden bay time
What a strong candidate does: Candidate: "Do cars ever sit in a bay without being worked on?" Interviewer: "Yes. About 30 percent of cars wait for parts, on average for four hours, and they stay in the bay while they wait."
Average bay hours per car: 2 + 0.3 × 4 = 3.2
Step 4: Real capacity
What a strong candidate does: With each car holding a bay for 3.2 hours on average, the workshop can finish far fewer cars than demand.
Cars per day in practice: 6 × 9 ÷ (2 + 0.3 × 4) = 16.88
Step 5: Cars lost each day
What a strong candidate does: Demand the workshop cannot serve.
Cars turned away per day: 24 - 6 × 9 ÷ (2 + 0.3 × 4) = 7.13
Step 6: Value of the lost work
What a strong candidate does: Interviewer: "Each service earns about ZAR 1,200 of contribution, and we open about 300 days a year."
Lost contribution (ZAR a year): (24 - 16.875) × 1,200 × 300 = 2,565,000
Step 7: Cost of the fix
What a strong candidate does: Candidate: "Instead of building bays, move cars that wait for parts to a parking area and keep the 40 most-used parts in stock." Interviewer: "A driver to move cars costs ZAR 180,000 a year, and the parts stock needs ZAR 250,000 one time."
Net gain in year one (ZAR): 2,565,000 - 180,000 - 250,000 = 2,135,000
The recommendation to listen for
At the end, say: "The CEO walks in. What is your recommendation?"
Do not build more bays; free the bays that already exist. First, in theory six bays can service 27 cars a day against demand of 24, so the bays are not too few. Second, cars waiting for parts hold a bay for four hours, which raises average bay time from 2 to 3.2 hours and cuts real capacity to about 17 cars a day, so about 7 cars a day are turned away. Third, moving waiting cars out of the bays and stocking common parts recovers that capacity for about ZAR 430,000 in year one, against about ZAR 2.6 million a year of lost contribution. Pilot the change in one workshop for a month, then roll it out.
Risks a strong answer names: Moving cars in and out of bays adds some handling time; Parts that are not stocked will still cause delays; At about 89 percent use, the bays will still build queues on busy days.
Next steps: Record for two weeks how long each car holds a bay and why; Agree faster delivery times with the two main parts suppliers; Pilot the parking area and parts stock in one workshop.
Strong versus weak
A strong answer
Compared capacity in theory with demand, noticed they did not explain the problem, found the hidden bay time, and valued a cheap fix against an expensive one.
A weak answer
Agreed with the owner to add two bays at each workshop, which costs far more and leaves cars still waiting for parts in the new bays.
Score the candidate
Score each criterion from 1 to 5. A 2 or a 4 sits between the descriptions.
This case has no exhibit. Score Exhibit reading on how the candidate used the data you gave them: did they pick out the number that matters and say what it means?
Total
0 out of 25
Score all five criteria to see the band and the feedback template.