MYP Maths: Fractions, Decimals & Percentages FAQ

Step-by-step approach:

1. Underline the numbers and note their current form (e.g., 3/8, 0.6, 45%).
2. Convert all values to decimals — it's usually the fastest common form for calculation.
3. Do the arithmetic the question asks for.
4. Convert the final answer back to whatever form the question specifies.
5. Sanity-check: does 45% of a number look roughly half of it? If your answer is wildly off, you've likely misplaced a decimal point.

Quick tip: to go fraction → percentage, multiply by 100 and simplify. To go percentage → fraction, write the percentage over 100 and reduce the fraction to lowest terms.

Worked example: Convert $0.\overline{6}$ to a fraction.

1. Let $x=\mathrm{0.666...}$
2. Multiply by 10: $10x = 6.666...$
3. Subtract: $10x - x = 6.666... - 0.666...$, so $9x = 6$
4. Solve: $x=6/9=2/3$

The same method handles longer repeating blocks — just multiply by 100 or 1000 depending on how many digits repeat.

Common mistake: A shirt goes from USD 40 to USD 50. Students often calculate 10/50, getting 20%. The correct working is 10/40 × 100 = 25% increase, because the original price is the base.

Quick tip: for reverse percentage problems (finding the original price after a discount), don't just add the percentage back onto the sale price — divide by (1 − discount rate) instead.

3 things to check before your next test:

1. Can you convert any fraction with a denominator up to 20 to a decimal without a calculator?
2. Do you know your common fraction-decimal-percentage equivalents (1/4, 1/3, 1/8, etc.) from memory?
3. Can you spot which number is the 'original' in a percentage-change question within 5 seconds of reading it?

According to the IB's MYP: Mathematics guide, Criterion D specifically rewards students who identify relevant elements of a real problem, select appropriate mathematical strategies, and justify the reasonableness of their solution — not just the final numeric answer.

Even with a calculator allowed, examiners still want the method shown — a correct final percentage with no working typically earns partial credit at best under Criterion A.

Worked example: USD 2,000 invested at 5% annual interest for 3 years, compounded annually.

$A=2000(1+0.05{)}^3=2000\times 1.157625=2315.25$

So the investment grows to USD 2,315.25 — the interest earned is USD 315.25. Students often mistakenly multiply 2000 × 0.05 × 3 (simple interest), which gives a smaller, incorrect USD 300.

A typical Criterion D task might describe a sale with successive discounts (e.g., 20% off, then a further 10% off the reduced price) and ask you to justify whether a shopkeeper's claim of '30% off' is mathematically accurate — it isn't, and spotting why is the actual skill being tested.

Worked example: An item costs USD 100. After 20% off: USD 80. After a further 10% off USD 80: USD 72. Total discount is 28%, not 30%. This is a favourite MYP examiner trap because it looks like simple addition but requires sequential calculation.

I've taught this for years, and the pattern is consistent: students who can convert 3/8 to a decimal instantly still stall on a two-line worded problem because they're not sure which number to use as the 'original' amount. That's a reading-and-reasoning skill, not a calculator skill, and it needs targeted worded-problem practice, not more drilling of bare conversions.

On RevisionPrep, MYP Mathematics Topical Worksheets on Number cover fraction, decimal and percentage conversions with worked solutions, so your child can check their own working step-by-step rather than only finding out they made a mistake once a mock paper is marked.