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Smartphones sold per year in a country (segmented, with replacement and first-time buyers)

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.

Estimate how many new smartphones are sold per year in a middle-income country of 100,000,000 people.

Format note: Assume the population splits into about 18 million children under 12 (not counted as buyers), 70 million people aged 12 to 59, and 12 million aged 60 and over.

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: New phones only, or second-hand phones too?
Answer: New phones only.
If asked: Is this a mature market where most adults already own a smartphone?
Answer: Yes, ownership is high and still growing slowly.

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.

In a mature market most sales are replacements, so my hypothesis is that sales are roughly owners divided by the years between replacements, plus a smaller number of first-time buyers.

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.

Yearly sales = replacement purchases + first-time buyers
- Replacements, ages 12 to 59: owners / 3 years
- Replacements, ages 60 and over: owners / 4 years
- First-time buyers: growth in ownership

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: Owners aged 12 to 59
What a strong candidate does: Assume about 90 percent of this group owns a smartphone.
Owners aged 12 to 59: 70,000,000 × 0.9 = 63,000,000
Step 2: Owners aged 60 and over
What a strong candidate does: Assume about 50 percent of this group owns one.
Owners aged 60 and over: 12,000,000 × 0.5 = 6,000,000
Step 3: Replacement purchases
What a strong candidate does: The younger group replaces about every 3 years, the older group about every 4 years.
Replacement phones per year: 63,000,000 ÷ 3 + 6,000,000 ÷ 4 = 22,500,000
Step 4: Add first-time buyers
What a strong candidate does: Ownership is still growing slowly: assume about 1 million people buy their first smartphone each year.
Total phones per year: 22,500,000 + 1,000,000 = 23,500,000
Step 5: Sanity-check against outside anchors
What a strong candidate does: Our answer is about 0.24 phones per person per year. India buys roughly 150 million smartphones a year for about 1.4 billion people (about 0.1 per person), and the US roughly 130 million for about 340 million (about 0.4). A middle-income country with high ownership should sit between them, and it does.
Phones per person per year: 23,500,000 ÷ 100,000,000 = 0.235

The recommendation to listen for

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

About 23.5 million phones a year. First, about 63 million owners aged 12 to 59 who replace every 3 years buy about 21 million phones. Second, about 6 million older owners who replace every 4 years add about 1.5 million. Third, about 1 million first-time buyers complete the total, which is about 0.24 phones per person, between India and the US as expected. Candidates who size only first-time buyers badly underestimate, and candidates who size only replacements miss the growth.

Risks a strong answer names: The replacement cycle is the swing assumption; a 3.5-year cycle for the younger group would lower sales by about 3 million.

Next steps: Test the answer with a 3.5-year cycle and state the range.

Strong versus weak

A strong answer

Segmented by age where behavior differs, included both replacements and first-time buyers, and checked the result against two real countries.

A weak answer

Multiplied the whole population, babies included, by one ownership rate and never checked the answer against anything.

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?

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.