CaseStar field guide · Red Rock math
Red Rock Study math: every calculation, worked
Red Rock never asks for advanced math. It asks for eight or nine small calculations done right the first time, under a clock you share with six mini-cases. Here is each skill, a worked example written for this guide, and the trap it usually hides.
1. Percentage change: divide by the start.
Change ÷ starting value. Everything else in Red Rock builds on getting this denominator right.
Worked example
Did the count grow by 20% or 25%?
A survey counted 120 nesting pairs in 2023 and 150 in 2024. What is the percentage increase?
Show the working
Change = 150 − 120 = 30. Divide by the start: 30 ÷ 120 = 25.00%. Dividing by the end value (30 ÷ 150 = 20%) answers “by what share would it have to fall to get back”, a different question.
Trap: a decline and a recovery of the same percentage do not cancel. Down 20% then up 20% leaves you at 96% of the start.
2. Percentage points vs percent.
When both numbers are already percentages, the difference between them is in percentage points. The relative change is a separate number.
Worked example
The share of protected land
Protected land rose from 14.4% of the island to 18.0%. By how many percentage points did it rise, and by what percent?
Show the working
Points: 18.0 − 14.4 = 3.6 percentage points. Percent: 3.6 ÷ 14.4 = 25.00%. Both are true statements; the report blank tells you which one it wants, so read its unit first.
4. Weighted averages.
When groups differ in size, the overall average is total ÷ total, not the average of the group averages.
Worked example
Reserve-wide density
Three zones hold deer at 40, 25 and 10 per km² and cover 50, 120 and 30 km². What is the density for the whole reserve?
Show the working
Animals: 40 × 50 + 25 × 120 + 10 × 30 = 5,300. Area: 200 km². Density = 5,300 ÷ 200 = 26.50 per km². The simple average (40 + 25 + 10) ÷ 3 = 25.00 treats a 30 km² zone as if it were as big as a 120 km² one.
The same logic applies to growth rates: the growth of a combined population is the change in the total divided by the starting total, not the average of each species’ growth rate. If a question asks for “the overall rate”, compute it from the totals.
5. Growth over several periods and CAGR.
Compound annual growth rate: (end ÷ start)1/years − 1. It is the constant yearly rate that gets you from start to end.
Worked example
Four years of growth
A colony grew from 800 in 2020 to 1,000 in 2024. What is its compound annual growth rate?
Show the working
Total growth: 1,000 ÷ 800 = 1.25, so +25% over four years. CAGR = 1.251/4 − 1 ≈ 5.74% per year.
The naive answer, 25% ÷ 4 = 6.25%, ignores compounding. Without a root key, test candidates: 1.0574 ≈ 1.248 and 1.064 ≈ 1.262, so the answer is just under 5.8%.
Projection trap: “If the same growth continues” means apply the same rate, not add the same number of animals.
6. Basic probability.
Independent events multiply; mutually exclusive events add. Expected value is probability × outcome, summed.
Worked example
Two tagged animals
30% of the island’s foxes carry a tracking tag. A ranger sees two foxes at random. What is the chance both are tagged, and that at least one is?
Show the working
The two sightings are independent: 0.30 × 0.30 = 9.00% for both tagged. At least one tagged is 1 − (0.70 × 0.70) = 1 − 0.49 = 51.00%, which is faster than adding the three cases.
7. Choosing the chart.
The report ends with one chart choice you cannot undo, and chart questions also appear among the mini-cases. Match the chart to the message:
| Message | Chart | Why not the others |
|---|---|---|
| Change over time | Line | Bars work for a few periods but hide the trend and crossovers. |
| Parts of one whole, one period | Pie or 100% stacked bar | A line has no whole to divide. |
| Comparing categories | Bar | A pie makes similar sizes hard to rank. |
| Relationship between two measures | Scatter | Bars show one measure at a time. |
| From one total to another | Waterfall | Only a waterfall shows each step’s contribution. |
Habits that save marks.
- Write the formula in words first. “Otters 2024 ÷ total 2024” catches wrong periods before you calculate.
- Estimate, then calculate. A one-second estimate catches a misplaced decimal.
- Round once. The study wants two decimal places; round only the final answer.
- Reuse, do not recompute. Note each result with its label and reuse it in later questions.
- Check the sum. Shares add to 100%, parts add to the total.
Put these into practice on a clock: the McKinsey Red Rock simulator starts with a free eight-minute sample, and the Red Rock Study guide explains where each calculation appears.
Common questions
How hard is the math in Red Rock?
The operations are school-level: percentages, ratios, averages and growth. The difficulty is doing them accurately on a shared 35-minute clock, with the right denominator, period and unit, and without rounding too early.
Is there a calculator in Red Rock?
Candidates report a built-in calculator that unlocks in the Analysis phase and stays available for the mini-cases. It is not available during Investigation. You may not use your own calculator or other tools.
How many decimal places should I use?
The study carries an instruction to round to two decimal places. Keep full precision in intermediate steps and round once, at the end. The mini-cases reportedly carry no rounding instruction; CaseStar’s practice cases state the precision they are marked to.
Do I need to know CAGR for McKinsey Solve?
It helps. Growth over several periods appears in candidate reports, and the classic trap is to divide total growth by the number of years instead of compounding. Knowing the difference is enough; exact nth roots are rarely needed.
What is the difference between percent and percentage points?
Percentage points measure the gap between two percentages (15% − 12% = 3 points). Percent change measures that gap relative to the start (3 ÷ 12 = 25%). Red Rock report blanks often specify which one they want.
Sources and limits
- McKinsey: Solve assessment overview and integrity rules
- McKinsey: Solve FAQ (August 2025)
- PrepLounge: “Didn’t perform well in Red Rock, did well in Sea Wolf”
- PrepLounge: McKinsey Solve, June 2026, Paris
- PrepLounge: Advice for McKinsey Solve
- PrepLounge: Advice to succeed on Redrock
- CaseStar: what changed in McKinsey Solve in 2026 (dated change log with confidence labels)
CaseStar (casestar.io) is an independent practice platform for consulting recruiting: timed McKinsey Solve simulators, including Red Rock, and voice case interviews. It is not affiliated with McKinsey & Company.
McKinsey publishes only a broad description of Solve. Details labelled as candidate-reported come from debriefs and can differ between candidates, regions and test versions. Worked examples on this page are original CaseStar exercises, not assessment questions. Your invitation and recruiter take precedence.