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Time Calculations

FoundationChallengeAQAEdexcelOCR

Get to grips with time calculations using these Foundation GCSE Maths practice questions. The worksheet focuses on calculating with time, and the accompanying mark scheme breaks down each solution clearly. Suitable for AQA, Edexcel and OCR. Download the questions and answers for free. There are 60 minutes in an hour, not 100 — convert carefully.

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Topic overview

Time calculations use base sixty rather than base ten, which is why they cause so many errors. There are \(60\) seconds in a minute and \(60\) minutes in an hour.

The classic mistake is treating times as decimals. Half an hour is \(30\) minutes, written as \(0.5\) hours, not \(0.30\). Similarly \(1\) hour \(45\) minutes is \(1.75\) hours, not \(1.45\).

When adding times, deal with the minutes first and carry an hour whenever the total reaches \(60\). Counting on from the start time is usually more reliable than subtracting, particularly when the interval crosses an hour boundary.

Revision notes

Converting minutes to hours

Divide the minutes by \(60\), not by \(100\).

So \(45\) minutes is \(45 \div 60 = 0.75\) hours, and \(20\) minutes is \(0.333\) hours. This matters whenever a speed calculation needs the time in hours.

Adding and subtracting times

Add the minutes first, carrying an hour whenever they reach \(60\).

So \(2\) hours \(40\) minutes plus \(1\) hour \(35\) minutes gives \(75\) minutes, which is \(1\) hour \(15\), making \(4\) hours \(15\) minutes in total.

Counting on across an hour

To find the interval between two times, count on from the earlier time to the next whole hour, then to the target.

From \(14{:}45\) to \(16{:}20\): \(15\) minutes takes you to \(15{:}00\), then \(1\) hour \(20\) more, giving \(1\) hour \(35\) minutes.

Key points

  • There are 60 minutes in an hour.
  • Divide minutes by 60 to convert to hours.
  • 45 minutes is 0.75 hours, not 0.45.
  • Carry an hour when minutes reach 60.
  • Count on rather than subtracting.
  • The 24-hour clock avoids am and pm confusion.

Worked examples

Example 1

Convert \(1\) hour \(30\) minutes to hours as a decimal.

Working

\[30 \div 60 = 0.5\]convert the minutes to a decimal
\[= 1.5 \text{ hours}\]add to the whole hour

Example 2

Add \(2\) hours \(50\) minutes and \(1\) hour \(25\) minutes.

Working

\[50 + 25 = 75 \text{ minutes}\]add the minutes first
\[75 = 1 \text{ hour } 15\]carry an hour
\[= 4 \text{ hours } 15 \text{ minutes}\]add the hours

Example 3

Find the time from \(13{:}40\) to \(15{:}25\).

Working

\[13{:}40 \to 14{:}00 = 20 \text{ minutes}\]count on to the next hour
\[14{:}00 \to 15{:}25 = 1 \text{ hour } 25\]count on to the target time
\[= 1 \text{ hour } 45 \text{ minutes}\]add the two parts

Common mistakes

  • Writing 1 hour 45 minutes as 1.45 hours.

    It is 1.75 hours. Divide the minutes by 60, not 100.

  • Subtracting times as if they were decimals.

    15.25 minus 13.40 is meaningless. Count on instead.

  • Forgetting to carry at 60 minutes.

    75 minutes is 1 hour 15, not 0 hours 75.

  • Confusing am and pm.

    Use the 24-hour clock to avoid ambiguity.

Exam tips

  • Count on rather than subtracting when finding an interval.
  • Divide minutes by 60 to get a decimal of an hour.
  • Use the 24-hour clock in your working.
  • Check the answer is sensible for the context.

Key terms

Interval
The time between two moments.
24-hour clock
A time system running from 00:00 to 23:59.
Carry
To convert 60 minutes into one hour.
Duration
How long something lasts.

Related topics

Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.