How we got there
Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.
Explain in another language · 11
1. What is the question asking?
Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: A fossil contains 25% of its original carbon-14. The half-life of C-14 is 5 730 years. Estimate its age.
Before any working — hunt first
Given
Numbers and units already in the sentence: 25% · 14 · 14 is · 5 · 730 years. Copy each one into a column. A name without a number is not given.
Missing
After you copy the given, the letter that still has no number is missing. The paper is pointing at: Estimate its age. Circle that word — that is the missing variable.
Find
We are required to find: Estimate its age. Write that as a symbol on the left, with a question mark, before you touch a formula.
Calculate
What we need to calculate: Each half-life halves the amount: 100. Then substitute the given, one line at a time, to get Estimate its age.
In other words — as if you have never seen this before
If you have never seen this before: Each half-life halves the amount: 100 → 50 → 25. We will name a method in words, write the formula with no numbers, then substitute one line at a time.
2. What do we already know?
25% left; half-life 5 730 years.
3. Which method — and why this one?
Each half-life halves the amount: 100 → 50 → 25.
4. Step-by-step working
01 / 05
Identify the given
100%→50%→25%=2 half-lives.
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Multiply
2×5 730=11 460 years.
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Step G — Sense-check the size
Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.
Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.
04 / 05
Step F — Box the answer with its unit
About 11 460 years old.
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.
5. The answer
About 11 460 years old.
Copy onto your answer sheet — in this order
Answer: About 11 460 years old.
6. Check it like a marker
C-14 is used for remains younger than about 50 000 years.
7. A trap first-timers fall into
First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”