This resource is about adding and subtracting multiple amounts of money involving 0.5 decimal fractions.
It also looks at amounts greater or less than given amounts. This serves as an indicator about students' understanding about the size of numbers.

The key thing to note is whether students can combine multiple amounts and how they handle adding or subtracting the 0.5 decimal fractions, i.e., whether they carry the "1" correctly.

Some of the common errors that students can make this resource are:

Common error

Likely reason

Next step

b)

Does not select one of
Comic book or Poster

Selects one answer rather than looking for all possible answers.

Encourage students to work out what the question is asking for: all possible answers when the question appears plural? or one answer only?

c)

Selects Watch ($9.50) as well as Poster and Comic book.

Does not keep the 1.50 subtracted from $10 and then identifies things to buy.

Encourage students to work out what the question is asking for. Students could explain as an expression, e.g., $10 - $1.50 = ? and then look for what is less than $8.50

b)

c)

d)

Selects onlyWatch ($9.50)

$2

Selects Poster ($7.50)

Does not carry the "1" correctly when subtracting the 1.50 from 10.00

Does not carry the "1" correctly when adding 1.50 + 7.50

Does not carry the "1" correctly when subtracting the 3.50 from 10.00

Recombining error

Students need to explore what happens when numbers with parts (decimal fractions) are added and subtracted. Using money and substituting two 50c for $1, and then adding and subtracting amounts should illustrate how recombination works.

c)

$9

Correctly recombines the 0.5 decimal fractions, but gives the amounts spent not the the change left.