Comparing fractions

More pieces, smaller pieces

20 minutes

Start the lesson

Before you read anything

Two identical strips of paper. Fold one into three and the other into twelve.

Tear one piece off each. Hold them side by side and say which is bigger — then look at the numbers you would write for them.

Two bars, the same length

Two bars of one length. One cut into three, one into twelve, and one piece shaded on each. Twelve is the bigger number and a twelfth is the smaller piece.

The numbers point the wrong way

$\frac{1}{12}$ has the bigger number in it and is the smaller amount. That is not a trick and it is not an exception: the bottom number counts the pieces the bar was cut into, and twelve pieces have to fit where three did.

:x nutshell-bigger-bottom

More pieces in the same whole, so each piece is smaller. The bottom number is a size, and it runs backwards.


The most reliable wrong answer in primary maths

Musa

said $\frac{1}{8}$ beats $\frac{1}{5}$ because 8 beats 5

Musa read the bottom number as an amount. It is not one — it is an instruction about how finely to cut, and finer cutting makes smaller pieces every time.

A :bigger bottom number always means a smaller slice, as long as the wholes are the same size.

As long as the wholes are the same

Half of a biscuit is less than a quarter of a cake. Comparing fractions only means anything when they are fractions of the same whole — and for the rest of this unit they always are.

Check yourself

Six to try

Five of these are one piece against one piece — say which bar was cut more finely. In the sixth the slices already match, so you only have to count them.

1

Which is biggest: $\frac{1}{3}$, $\frac{1}{5}$ or $\frac{1}{6}$?

2

Which is biggest: $\frac{1}{3}$, $\frac{1}{5}$ or $\frac{1}{12}$?

3

Which is biggest: $\frac{1}{2}$, $\frac{1}{10}$ or $\frac{1}{12}$?

4

Which is biggest: $\frac{1}{5}$, $\frac{1}{6}$ or $\frac{1}{12}$?

5

Which is biggest: $\frac{2}{10}$, $\frac{5}{10}$ or $\frac{8}{10}$?

6

Put these in order, smallest first.

Away from the screen

One piece against one piece Say which was cut more finely, then answer.
One piece against one piece, page 1 One piece against one piece, page 2 One piece against one piece, page 3

Open the PDF Save it to print

Show that you have it

The reason, and the order

One asks why, and one asks you to put four of them in order. Getting the order backwards is the mistake this whole lesson is about.

1

Why is $\frac{1}{10}$ smaller than $\frac{1}{4}$?

2

Put these in order, smallest first.