Using a calculator Revision Notes

When performing calculations, it is important not to round numbers during intermediate steps. Rounding too early can cause cumulative errors, leading to inaccurate final answers. Instead, keep all decimal places during working and only round the final result to the required degree of accuracy.

For example, if you calculate 12.345 × 6.789, do not round 12.345 to 12.3 before multiplying. Multiply the full numbers first, then round the final product as needed.

• Keep full calculator precision during intermediate steps.
• Round only the final answer to the required decimal places or significant figures.
• This practice improves accuracy and reliability of results.

Calculators work best with decimal numbers, so time given in hours and minutes must be converted into decimal hours before input.

To convert minutes to a decimal fraction of an hour, divide the minutes by 60.

For example, 2 hours 30 minutes is converted as: 2 + (30 ÷ 60) = 2 + 0.5 = 2.5 hours. Enter 2.5 into the calculator, not 2:30 or 2.30, which the calculator interprets incorrectly.

• Convert minutes to decimal by dividing by 60.
• Add the decimal fraction to the whole hours.
• Input the decimal number into the calculator for accurate calculations.

When dealing with money, a calculator may display numbers like 4.8, which can be ambiguous.

In monetary terms, 4.8 usually means £4.80, not £4.08 or £48.

Always interpret the decimal part as tenths and hundredths of the currency unit, corresponding to pence or cents, so 4.8 = £4.80 = 4 pounds and 80 pence.

• Recognise that one decimal place in money represents tens of pence.
• Two decimal places represent pounds and pence exactly.
• Use this understanding to write answers correctly in monetary format.

Time displayed as a decimal on a calculator must be converted back into hours and minutes for meaningful interpretation.

For example, 3.25 hours means 3 hours plus 0.25 of an hour.

Since 0.25 hour = 0.25 × 60 minutes = 15 minutes, 3.25 hours equals 3 hours 15 minutes, not 3 hours 25 minutes (which would be 3.4167 hours).

• Multiply the decimal part by 60 to find minutes.
• Separate the whole number as hours.
• Convert decimal time back to hours and minutes for final answers.

Example 1: Calculate the total time if a task takes 1 hour 45 minutes and another takes 2 hours 30 minutes.

Convert times: 1 hour 45 minutes = 1 + 45/60 = 1.75 hours; 2 hours 30 minutes = 2.5 hours.

Add: 1.75 + 2.5 = 4.25 hours on the calculator (do not round). Convert back: 0.25 × 60 = 15 minutes, so total time = 4 hours 15 minutes.

• Always convert mixed units before calculation.
• Add decimal values directly on the calculator.
• Convert decimal results back to mixed units if required.