Calculate and show working
Calculate biological rates, ratios, percentages and magnifications with visible reasoning, consistent units and justified significant figures.
Cambridge International Biology 9700, 2028–2030. Paper 3 assesses practical skills in a supervised laboratory; these lessons support preparation but do not replace safe, supervised laboratory experience.
Before you begin
- Use biological vocabulary and basic calculations from the AS Biology course. Follow your school laboratory rules and teacher instructions for all practical work.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How should a candidate present a calculated value?
What you will learn
- Calculate a rate from a defined change and time.
- Convert units before combining measurements.
- Show steps and round only at the end to justified significant figures.
Show how a value was obtained
Write the equation, substitute the data and show the result with units. Define a rate clearly, such as oxygen volume per minute, before calculating it. Keep intermediate values unrounded where possible.
Convert measurements into compatible units before calculating. For a percentage change, state the original value as the denominator and distinguish percentage change from percentage difference.
Use significant figures deliberately
A calculated quantity should generally use the same number of significant figures as, or one more than, the least precise measurement used. Do not carry unjustified digits or round each intermediate step.
An answer without working may hide an incorrect operation or unit conversion. Clear reasoning lets another person follow and check the calculation.
Pause and explain
A specimen produces 9.6 cm³ in 3.0 min. What is its mean rate?
Worked example
A plant releases 7.2 cm³ oxygen in 4.0 minutes. Find the mean rate.
Show the worked solution
- Rate = volume ÷ time.
- 7.2 ÷ 4.0 = 1.8.
- Retain two significant figures and the unit.
Answer Mean oxygen production rate = 1.8 cm³ min⁻¹.
From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A solution changes from 40 to 52 units. Calculate the percentage increase.
Give me a hint
Use (new − original) ÷ original × 100%.
Compare my reasoning
- Increase = 52 − 40 = 12.
- Percentage increase = 12 ÷ 40 × 100.
- State 30%.
The increase is 12 units; divide it by the original 40 and multiply by 100, giving a 30% increase.
Look for these in your work
- Increase calculated
- Original used as denominator
A 0.06 mm tissue layer is viewed at ×250. Calculate its image size in millimetres.
Give me a hint
Image size = actual size × magnification.
Compare my reasoning
- 0.06 mm × 250 = 15 mm.
- Keep the unit in millimetres.
Image size = actual size × magnification = 0.06 mm × 250 = 15 mm.
Look for these in your work
- Correct operation
- Unit retained
A student’s calculated rate is 0.34782 g min⁻¹ from measurements recorded to 2 significant figures. How should it be reported?
Give me a hint
Do not claim more precision than the input data justify.
Compare my reasoning
- Round to 2 significant figures.
- Keep the rate unit.
Report 0.35 g min⁻¹, unless the question requires a stated alternative precision.
Look for these in your work
- Appropriate rounding
- Unit included
Common mistakes
- Writing a rate without its “per time” unit.
- Rounding intermediate values before the final calculation.
- Confusing actual size with image size.
What can you explain now?
How should a candidate present a calculated value?
Compare with the explanation
Show the calculation steps and include a justified unit
This makes reasoning and precision assessable.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What are the three essentials of a biological calculation in a practical answer?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.