3.5.3 Dividing fractions

The meaning of an expression such as four division one divided by two is not immediately obvious, but a comparison with a more familiar expression, say six division two may help. six division two asks us to work out how many twos there are in 6 (the answer is 3). In exactly the same way, four division one divided by two asks how many halves there are in 4. Figure 3.2 illustrates this in terms of circles. Each circle contains two half-circles, and 4 circles therefore contain 8 half-circles. So

equation sequence part 1 four division one divided by two equals part 2 four multiplication two divided by one equals part 3 eight
Figure 3.2 Four circles each containing two half-circles.

This can be extended into a general rule:

To divide by a fraction, turn it upside down and multiply.

So

equation sequence part 1 five division eight divided by three equals part 2 five multiplication three divided by eight equals part 3 five multiplication three divided by eight equals part 4 15 divided by eight

And

Finally, remembering that 3 can be written as three divided by one

equation sequence part 1 one divided by two division three equals part 2 one divided by two division three divided by one equals part 3 one divided by two multiplication one divided by three equals part 4 one multiplication one divided by two multiplication three equals part 5 one divided by six

Question 3.7

Work out the following, leaving each answer as the simplest possible fraction:

    • a. two divided by seven division one divided by four
  • equation sequence part 1 two divided by seven division one divided by four equals part 2 two divided by seven multiplication four divided by one equals part 3 two multiplication four divided by seven multiplication one equals part 4 eight divided by seven

    • b. two divided by three division three divided by four
  • equation sequence part 1 two divided by three division three divided by four equals part 2 two divided by three multiplication four divided by three equals part 3 two multiplication four divided by three multiplication three equals part 4 eight divided by nine

    • c. three divided by four division five
  • equation sequence part 1 three divided by four division five equals part 2 three divided by four division five divided by one equals part 3 three divided by four multiplication one divided by five equals part 4 three multiplication one divided by four multiplication five equals part 5 three divided by 20

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4 Decimal numbers and decimal places