MYP Maths: Decimals & Percentages — What Students Need to Know

MYP 1-3 students need to convert fluently between fractions, decimals and percentages, round to significant figures and decimal places, calculate percentage increase/decrease, and apply this to real contexts like discounts, tax and interest. It falls under Criterion A (Knowing and Understanding) and appears constantly in Criterion D investigations too.

Divide the numerator by the denominator to get a decimal, then multiply by 100 to get a percentage. For 3/8, that's 3 ÷ 8 = 0.375, so 0.375 × 100 = 37.5%. This three-step chain — fraction, decimal, percentage — is the single most-tested conversion skill in MYP 1-2.

Find the actual change (new value minus original), divide by the original value, then multiply by 100. A price rising from 40 to 50 is a change of 10, so 10 ÷ 40 × 100 = 25% increase. Always divide by the ORIGINAL value, not the new one — that's the most common error.

Percentage change tells you how much a value moved, given both the before and after figures. Reverse percentages work backwards — you're given the final amount after an increase or decrease and need to find the original. Reverse problems trip up more students because you can't just multiply by the percentage directly.

Most struggles come from treating decimals and percentages as separate topics rather than one system of representing the same value. Students memorise a conversion rule without understanding why it works, so they can't adapt when a question phrases things unfamiliarly — like a reverse percentage or a compound change over two steps.

The top three: dividing by the new value instead of the original in percentage-change questions, forgetting to convert a percentage into its decimal form before multiplying, and rounding intermediate steps too early. Each one is fixable with a checklist habit rather than more practice questions alone.

Top bands (7-8) in Criterion A require you to apply mathematical concepts and skills in both familiar AND unfamiliar situations with a considerable degree of accuracy. For percentages, that means handling multi-step or reverse problems correctly, showing full working, and justifying which method you chose — not just getting the right number.

Because MYP assessment is built around real-life contexts — discounts, currency exchange, tax, interest, population growth — and percentages are the natural language for describing proportional change in all of them. Criterion D (Applying Mathematics in Real-Life Contexts) almost always involves at least one percentage calculation.

Simple interest is calculated only on the original amount each year, so the interest earned stays the same every year. Compound interest is calculated on the original amount plus any interest already earned, so it grows faster over time. MYP 2-3 students meet both, usually through savings-account or loan-style word problems.

The most useful thing you can do is turn everyday moments into practice — a supermarket discount sign, a restaurant bill with a service charge, a bank statement — and ask your child to calculate the actual numbers. This builds the real-life fluency MYP Criterion D rewards, far more than repeating worksheet drills.

Yes, in a real way. Percentages and proportional reasoning underpin DP topics like compound growth, statistics and financial mathematics in both Applications and Interpretation (AI) and Analysis and Approaches (AA). A shaky foundation in MYP 1-3 tends to resurface as a struggle with exponential functions and financial maths in DP Year 1.

Look for resources that mirror actual MYP command terms and criterion language — not generic percentage worksheets — since MYP assessment rewards justified method as much as the right answer. One-to-one tutoring is particularly effective here because reverse-percentage and multi-step problems need real-time correction of why a step is wrong, not just that it is.