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Decimal arithmetic is essential for real-world problem solving and calculator-based exam questions. Students need fluency with the four operations on decimals, place value understanding, and converting between decimals and other number forms.
Students make these errors again and again. Knowing them in advance gives you a head start.
Misaligning the decimal point during written addition or subtraction
Assuming 0.45 is larger than 0.5 because 45 is bigger than 5
Forgetting to adjust the decimal point after multiplying decimals
Insights pulled from Cambridge IGCSE (0580) examiner reports — the exact mistakes candidates make every year.
For time intervals crossing midnight, split into two parts: time until midnight + time after midnight. Remember an hour has 60 minutes, not 100.
Source: CIE 0580 · Feb/March 2024 · Paper 1 · Q5
For compound interest, the rate and the number of periods must MATCH. 0.25% per month over 5 years = 60 monthly periods (not 5). Don't silently convert to annual — apply the rate as given.
Source: CIE 0580 · Feb/March 2023 · Paper 4 · Q1a(iii)
When finding bounds, always give the final answer in the SAME UNITS as the variable in the question. If l is in metres, bounds must be in metres — convert at the end if your working was in cm.
Source: CIE 0580 · May/June 2023 · Paper 1 · Q18
Based on 3of 500+ insights extracted from CIE 0580 examiner reports (2018–2024).
This topic appears in the Cambridge IGCSE past papers. Our AI tutor can help with it on any question.
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Key number facts, rules, and formulae at a glance