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Time and money Cambridge IGCSE Mathematics revision

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In plain words

Time doesn't work in tens: there are 60 minutes in an hour, not 100. That one fact causes most of the mistakes. Money questions are mostly about spotting what is being asked: a profit, a discount, or which pack is the better buy.

Three things to know

  1. The 24-hour clock runs from 00 00 to 23 59: 3.40 pm is 15 40. To find how long something takes, count up to the next hour, then on.
  2. A decimal part of an hour is a fraction of 60 minutes: 0.25 h is 15 minutes, 0.5 h is 30, 0.75 h is 45.
  3. Profit = selling price − cost price. Percentage profit is the profit divided by the cost price, × 100. For a best buy, work out the price for the same amount of each.

Worked example

A bus leaves at 09 45 and arrives at 13 20. How long is the journey?

  1. From 09 45 to 10 00 is 15 minutes.
  2. From 10 00 to 13 20 is 3 hours 20 minutes.
  3. Altogether: 3 hours 35 minutes.

Tips and tricks

  • Don't subtract times as if they were decimals. 13.20 − 9.45 gives the wrong answer.
  • On a calculator, 2.25 hours means 2 and a quarter hours: 2 h 15 min, not 2 h 25 min.
5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Time and money: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. What is 15 40 on the 12-hour clock?
    • 5.40 am
    • 1.40 pm
    • 3.40 pm (the answer)
    • 5.40 pm

    Subtract 12 from the hours.

  2. What is 1.75 hours in hours and minutes?
    • 1 hour 75 minutes
    • 1 hour 45 minutes (the answer)
    • 1 hour 7.5 minutes
    • 2 hours 15 minutes

    0.75 of an hour is 45 minutes.

  3. How long is it from 08 50 to 11 15?
    • 2 hours 65 minutes
    • 3 hours 25 minutes
    • 2 hours 25 minutes (the answer)
    • 2 hours 35 minutes

    10 minutes to 09 00, then 2 hours 15 minutes.

  4. An item costs $80 and is sold for $100. What is the percentage profit?
    • 20%
    • 25% (the answer)
    • 80%
    • 125%

    20 ÷ 80 × 100.

  5. Which is the better buy: 3 bars for $1.20 or 5 bars for $1.90?
    • 3 bars, at 40 cents each
    • 5 bars, at 38 cents each (the answer)
    • they are the same
    • 3 bars, at 36 cents each

    $1.90 ÷ 5 = 38 cents, which beats 40 cents.

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