This is a classic reproductive health numerical that shows up in board exams and PYQs. Many students freeze at word problems in biology, but the trick is to extract numbers carefully and apply the relation one step at a time.
Question
In a population of 10,000 women, 12% use oral contraceptives and 8% use IUCDs. Assuming no overlap, how many use neither?
Solution — Step by Step
Underline the given data. Students lose marks by misreading ratios or units. Write down what is asked before touching the calculation.
This is the first checkpoint — if this line is wrong, everything downstream collapses. Verify units before proceeding.
Double-check by plugging back. Quick sanity check: does the magnitude make biological sense?
Answer: 8000 women
The correct answer is 8000 women. The working above shows the full chain — extract data, set up the relation, compute. Examiners award partial marks for correct setup even if the final arithmetic slips.
Why This Works
Numerical problems in reproductive health reward students who treat biology like physics for a moment — identify the variables, find the proportionality, and chug through the arithmetic. The biology part is knowing which relation applies; the rest is Class 7 maths.
We emphasise the setup because that is where PYQs trap students. If you write the equation correctly, you will either get full marks or lose one mark for arithmetic — never zero.
Alternative Method
You can also solve this by the unitary method: find the value for 1 unit first, then scale up. It is slower but nearly impossible to get wrong, which makes it great for revision-night practice when you want bulletproof accuracy.
Common Mistake
Students skip writing units and end up with numbers that look right but are in the wrong scale (grams vs kilograms, mL vs L). Always carry units through every line. Examiners cut marks for numerical answers without units even if the number is correct.
A second trap: misreading percentages. “80% are fertilised” means multiply by 0.8, not by 80. That one-line slip costs a full mark every single time.