What are the three AP Environmental Science free-response questions?

Section II has three free-response questions in 1 hour 10 minutes, worth 40% of the exam. The first asks you to design an investigation (identify a hypothesis, variables and method). The second asks you to analyze an environmental problem and propose a solution, including at least one calculation. The third asks you to analyze an environmental problem and propose a solution using a provided data set, again including calculations. Every free-response question rewards specific, justified answers and shown working.

How do I earn the calculation points on an APES free-response question?

Always show your work. You earn points for the setup (the correct equation or arrangement with units), for the substitution of the right numbers, and for the correct final answer with units. Use dimensional analysis so units cancel, label every number, and box your final answer. A correct answer with no working may not earn full points, and a wrong final answer can still earn the setup and substitution points if the working is shown.

What is the rule of 70 in AP Environmental Science?

The rule of 70 estimates the doubling time of any quantity growing at a constant percentage rate. Doubling time in years is approximately 70 divided by the percentage growth rate per year. For example, a population growing at 2% per year doubles in about 70 divided by 2, which is 35 years. It is used for population, consumption and any exponentially growing quantity.

How do I calculate percentage change on the APES exam?

Percentage change equals the new value minus the old value, divided by the old value, all multiplied by 100. A positive result is an increase and a negative result is a decrease. Always divide by the original (old) value, not the new one, and keep the sign so you can state whether the quantity rose or fell.

How is the 10% rule used in calculations?

The 10% rule states that about 10% of the energy in one trophic level passes to the next. To find the energy at a higher level, multiply by 0.10 once for each trophic transfer. For example, if producers store 10,000 kcal, secondary consumers (two transfers up) receive about 10,000 times 0.10 times 0.10, which is 100 kcal. Count the transfers carefully.

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How to answer AP Environmental Science free-response questions and do environmental calculations: a complete exam-technique guide

A deep-dive AP Environmental Science exam-technique guide to the three free-response question types, how to read APES task verbs, and how to lay out the core environmental calculations the exam repeats: percentage change, dimensional analysis, scientific notation, the rule of 70, primary productivity and the 10% rule, with worked steps.

Generated by Claude Opus 4.818 min readAPES-FRQUpdated 2026-06-04

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What the APES exam actually rewards
The three free-response questions
Reading the task verbs
The core environmental calculations
Check your knowledge

What the APES exam actually rewards

AP Environmental Science is won or lost on two skills: writing specific, justified free-response answers, and laying out clear calculations that earn every point. The content is broad, but the marking is predictable. Examiners give points for precise statements that match the task verb, and for calculations where the setup, substitution and final answer (with units) are all shown. This guide covers the three free-response question types and the core calculations the exam repeats, with worked steps you can copy under pressure.

It ties together the matching dot-point pages, each with its own practice questions: introduction to ecosystems, primary productivity, energy flow and the 10% rule, the carbon cycle and introduction to biodiversity.

The three free-response questions

Section II has three free-response questions in 1 hour 10 minutes, worth 40% of the exam:

Question 1, design an investigation. You are given a scenario and asked to identify a testable hypothesis, the independent and dependent variables, a control, and a method, and to explain how you would analyze the results. Be specific: name the variable you would change and the variable you would measure.
Question 2, analyze an environmental problem and propose a solution. You explain the cause and effects of a problem and propose a realistic solution, including at least one calculation.
Question 3, analyze a data set and propose a solution. You read a provided table or graph, perform calculations from it, and use the results to justify a solution.

Every question is broken into lettered parts (a, b, c, ...), each worth a point or two, so answer each part separately and label it.

Reading the task verbs

APES marking is tightly tied to the task verb in each part. Match your answer to the verb:

Identify / State: give a brief, specific answer (a word or phrase). Do not over-explain.
Describe: give the characteristics of something; say what it is or how it works.
Explain: give the reason or mechanism; use "because" to show cause and effect.
Calculate: show the working and give a numerical answer with units.
Justify / Make a claim: state a position and support it with evidence and reasoning.
Propose a solution: give a specific, realistic action and say why it would work.

A common reason for lost marks is answering a different verb than the one asked, for example describing when the question said explain.

The core environmental calculations

The exam reuses a small set of calculations. Master the layout of each.

Percentages and percentage change

A percentage is a part out of 100. Percentage change compares a new value with an old one:

\text{percentage change} = \frac{\text{new} - \text{old}}{\text{old}} \times 100

Always divide by the old (original) value, and keep the sign: positive means an increase, negative means a decrease.

Dimensional analysis (unit conversions)

Dimensional analysis converts units by multiplying by fractions equal to one, so that unwanted units cancel:

\text{quantity} \times \frac{\text{wanted unit}}{\text{given unit}}

Write every unit and cancel them diagonally; if the units do not cancel to the answer you want, the setup is wrong. This is the single most useful skill on the exam.

Scientific notation

Express very large or very small numbers as a coefficient between 1 and 10 times a power of ten, for example $6.5 \times 10^{6}$ . Multiply the coefficients and add the exponents when multiplying; divide the coefficients and subtract the exponents when dividing.

The rule of 70

The rule of 70 estimates the doubling time of a quantity growing at a constant percentage rate:

\text{doubling time (years)} \approx \frac{70}{\text{growth rate (\% per year)}}

For example, a quantity growing at 2% per year doubles in about $70 / 2 = 35$ years.

Primary productivity

From Topic 1.8, net primary productivity is gross productivity minus respiration:

\text{NPP} = \text{GPP} - \text{respiration}

The 10% rule

From Topic 1.10, about 10% of energy passes between trophic levels. Multiply by 0.10 once per transfer; secondary consumers (two transfers above producers) receive about $0.10 \times 0.10 = 0.01$ of the producers' energy.

AP Environmental Science Section II has three free-response questions worth 40%: design an investigation, analyze a problem and propose a solution (with a calculation), and analyze a data set and propose a solution. Match your answer to the task verb (Identify, Describe, Explain, Calculate, Justify, Propose). For every calculation, show the equation, the substitution with units, and the boxed final answer with units, so you earn the setup and substitution points even if the arithmetic slips. Master the repeated calculations: percentage change (new minus old, over old, times 100), dimensional analysis, scientific notation, the rule of 70 for doubling time (70 over the percentage growth rate), net primary productivity (GPP minus respiration), and the 10% rule (multiply by 0.10 per trophic transfer).

Check your knowledge

A mix of technique and calculation questions. Attempt them under timed conditions, then check against the solutions.

State the formula for percentage change and what value you divide by. (2 marks)
A population of 5,000 grows to 5,400 in one year. Calculate the percentage change. (2 marks)
Use the rule of 70 to find the doubling time of a population growing at 3.5% per year. (2 marks)
Convert 2.5 kilometers to centimeters using dimensional analysis. (2 marks)
Calculate the NPP of an ecosystem with a GPP of 18,000 kcal/m^2/year where producers respire 7,000 kcal/m^2/year. (2 marks)
Producers store 60,000 kcal/m^2/year. Using the 10% rule, calculate the energy available to secondary consumers. (2 marks)
Explain the difference between what the task verbs "describe" and "explain" require. (2 marks)
State the three things you should write to earn full points on a calculation question. (2 marks)

Solutions

Q1: Percentage change = (new minus old) divided by old, times 100; you divide by the old (original) value.
Q2: $\dfrac{5{,}400 - 5{,}000}{5{,}000} \times 100 = \dfrac{400}{5{,}000} \times 100 = 8\%$ increase.
Q3: Doubling time $\approx \dfrac{70}{3.5} = 20$ years.
Q4: $2.5\ \text{km} \times \dfrac{1{,}000\ \text{m}}{1\ \text{km}} \times \dfrac{100\ \text{cm}}{1\ \text{m}} = 250{,}000\ \text{cm} = 2.5 \times 10^{5}\ \text{cm}$ .
Q5: $\text{NPP} = \text{GPP} - \text{respiration} = 18{,}000 - 7{,}000 = 11{,}000$ kcal/m^2/year.
Q6: Two transfers up: $60{,}000 \times 0.10 \times 0.10 = 600$ kcal/m^2/year.
Q7: "Describe" asks for the characteristics or features of something (what it is or how it works); "explain" asks for the reason or mechanism (why it happens, using cause and effect).
Q8: The equation (setup with units), the substitution of the correct numbers, and the final answer with units.

Sources & how we know this

AP Environmental Science Course and Exam Description — College Board (2020)