Using a calculator
Section: Number 2 | Syllabus: Cambridge IGCSE Mathematics (0580)
Using a Calculator Efficiently and Correctly
A calculator only gives a correct final answer if the values are entered accurately and rounded only at the very end. Reading the display correctly matters just as much as the calculation itself.
- Never round a value part-way through a calculation - keep full accuracy throughout, and round only the final answer
- Mixed units (like hours and minutes) must be converted to a single decimal quantity before being entered, e.g. minutes are converted to a fraction of an hour
- The calculator display must be interpreted according to what the number represents - the same decimal means something different for money than it does for time
Worked Example: Why Early Rounding Causes Errors
- Question: Work out (8.6 × 5.3) ÷ 1.9, correct to 2 decimal places.
- Correct method: Keep full accuracy: 8.6 × 5.3 = 45.58, then 45.58 ÷ 1.9 = 23.9894...≈ 23.99
- Incorrect method: Rounding 45.58 to 45.6 first, then dividing: 45.6 ÷ 1.9 = 24.00 - a different, incorrect answer
Worked Example: Entering Mixed Units
- Question: Enter 3 hours 45 minutes as a decimal number of hours.
- Step 1: Convert the minutes to a fraction of an hour: 45 ÷ 60 = 0.75
- Answer: 3.75 hours
Worked Example: Interpreting the Calculator Display
- A money calculation displays 7.5 - this means 7.50, not 7.05 or 7.5 read as "seven point five cents"
- A time calculation displays 5.4 hours - the 0.4 is a fraction of an hour, not 40 minutes: 0.4 × 60 = 24 minutes, so the answer is 5 hours 24 minutes
Common Mistakes
MistakeRounding a value in the middle of a multi-step calculation before continuing
Fixkeep every digit the calculator gives until the very last step - early rounding can change the final rounded answer, as in 23.99 versus 24.00 above
MistakeWriting a money answer without 2 decimal places, e.g. writing 4.8 instead of 4.80
Fixmoney should always be written to 2 decimal places - a calculator display of 4.8 means 4.80
MistakeReading a decimal time display as if the digits after the point were minutes, e.g. reading 5.4 hours as 5 hours 40 minutes
Fixmultiply the decimal part by 60 to find the minutes: 5.4 hours = 5 hours + (0.4 × 60) minutes = 5 hours 24 minutes
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