Primary 4 · Mathematics
P4 decimals
One habit costs most of the marks here: thinking the longer decimal is the bigger one. Here is why 0.45 is not bigger than 0.5, and a one-line way to check it at home.
- Level
- P4
- Usually taught
- Term 3
- Covers
- 8 concepts
What changes at P4
In Primary 4, your child meets decimals: tenths and hundredths, reading them, placing them on a number line, comparing, ordering, adding and rounding. It is the first time the place-value idea reaches to the right of the ones.
The slip that costs marks is comparing decimals like whole numbers. 0.45 has more digits than 0.5, so it *looks* bigger, the way 45 is bigger than 5. But digit count is not size: you compare place by place, starting from the tenths.
The sentence that costs the mark
Which is bigger, 0.45 or 0.5?
What the working usually does
digits0.45 vs 0.5=0.45
Compares them like whole numbers: 45 beats 5, so it picks 0.45. But more digits does not mean a bigger number.
What the question asked
tenths0.45 vs 0.5=0.5
Lines the points up and compares the tenths first: 5 tenths beats 4 tenths, so 0.5 is bigger. 0.5 is the same as 0.50.
0.45 does have more digits, and for whole numbers more digits does mean bigger, so the habit transfers. Lining up the decimal points shows the truth: 0.5 is 0.50, which is 5 tenths against 4, so 0.5 wins before you reach the hundredths.
| Point | Value |
|---|---|
| 0.45 | 0.45 |
| Point 2 | 0.5 |
The same question, worked properly
Do not count digits
line up the decimal points0.45 has more digits than 0.5, but that does not make it bigger. Line the decimal points up and fill a placeholder zero so both have the same length: 0.5 = 0.50.
Compare the tenths first
5 tenths vs 4 tenthsThis is the step that fails. Start from the biggest place, the tenths. 0.50 has 5 tenths, 0.45 has 4, so 0.50 is already bigger before the hundredths matter. Comparing 45 against 5 is the slip.
So 0.5 is the bigger number
0.50 > 0.450.50 is bigger than 0.45, so 0.5 is the larger number. In money it is obvious: half a dollar beats forty-five cents.
How can you tell it has clicked?
Write 0.45 and 0.5 and ask which is bigger. If they line the points up, say 0.5 is 0.50 and compare the tenths, they have it. If they say 0.45 because "45 is more than 5", that is the exact slip.
The money version is the quickest check: would they rather have $0.50 or $0.45? Every child picks the fifty cents, which is the same as saying 0.5 is bigger.
One thing to try tonight
Use coins. $0.50 versus $0.45, even though "45" looks like the bigger number. Ask which they would rather have, then line the two amounts up and count the tenths together: 5 against 4.
Then try pairs like 0.3 and 0.28, or 0.7 and 0.65. Each time, line up the points, fill a zero if one is shorter, and compare from the left. Never count digits.
What your child will meet in this topic
- Decimals
- Decimal Place Value
- Comparing Decimals
- Adding Decimals
- Multiplying Decimals
- Dividing Decimals
- Rounding Decimals
- Fractions and Decimals
Most Singapore schools introduce this in Term 3, though schools do differ by a term either way. If your child’s school runs a different order, you can set it in the schedule on your dashboard.
What it leads to
Comparing and ordering decimals underpins money, measurement and every later decimal topic: adding, subtracting, and the multiplying and rounding that come in P5. A shaky comparison here wobbles all of them.
A child who compares place by place tends to line decimals up correctly when adding and subtracting too, which is where a misread place value quietly loses marks.
The same topic in other years
It comes back each year and gets harder, so it helps to see what came before and what is coming.
More in Primary 4
Other topics in the same year, so you can see what sits either side of this one.
Struggling with the working rather than the topic itself? How to help your child with problem sums
See how your child is doing on this topic
StudyLah marks each step separately, so you can see whether it was the setup or the arithmetic that cost the mark.