Primary 3 Maths — One-Place Decimals: a Tutor Lesson-Prep Guide (tenths, number line, aligned columns)

2026-11-10

Tutor lesson-prep guides · Primary maths

A pupil writes 2.4 plus 1.7 as 3.11, because he treated the digits after the point as a separate sum. This lesson teaches one action: line up the decimal points before adding.

Objectives — By the end the pupil can: (1) say that 0.1 is one tenth of a whole and mark 0.4 and 0.7 on a number line; (2) write 2.4 plus 1.7 in columns with the points aligned, add digit by digit to get 4.1, and explain that four tenths plus seven tenths is eleven tenths so one must be carried; (3) write 3 dollars 5 cents as 3.5 dollars and 47 millimetres as 4.7 centimetres, saying both are the same quantity.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Meaning of one tenth	A strip is cut into ten equal parts. What is one part?	One tenth, written 1/10
Numerator and denominator	In 3/10, what is the numerator?	3
Length conversion	How many millimetres are in one centimetre?	Ten

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Error diagnosis	Write 2.4 plus 1.7 and watch whether the answer comes out as 3.11	Works it and explains each step
5-12 min	Cut ten parts	Cut a paper strip into ten equal parts and ask for the value of one	Says one part is 0.1 and seven parts are 0.7
12-22 min	Three-act teaching	Deliver the scripts on tenths, the number line and aligned columns	Repeats: the decimal points must sit above each other
22-32 min	Worked examples	Model one reading, one comparison, one column sum and one conversion	Copies one and checks the point position
32-40 min	Independent practice	Hand out four questions, prompting only with draw a number line	Works alone and keeps the column working
40-45 min	Exit check	Collect the work and note whether the point or the carrying failed	Says which part went wrong

03 Teaching script (read it out)

Scene 1: Tenths: 0.1 is one of ten equal parts

Cut a strip into ten equal parts. One part is one tenth, written 1/10 as a fraction and 0.1 as a decimal. Three parts are 3/10, that is 0.3, and all ten parts together make 1, written 1.0. The place after the point is called the tenths place, so 0.7 means seven tenths, not simply seven. Have the pupil cut the strip himself and then say: one part is 0.1, seven parts are 0.7.

0.1 is one of ten equal parts of one whole.

Scene 2: The number line: decimals sit between 0 and 1

Draw a line from 0 to 1 and divide it into ten equal steps. The first mark is 0.1, the fourth 0.4, the seventh 0.7, and the tenth is 1. Mark 0.4 and 0.7 and ask which is further right; the one further right is always larger, and that rule carries over from whole numbers. Now place 0.9 and 1.2: 0.9 is still left of 1, while 1.2 is past it, so 1.2 is larger. A pupil who answers 0.9 because nine is bigger than two needs to go back to the line and look at position.

Further right on the number line is larger.

Scene 3: Column form: align the points, then add digit by digit

Write 2.4 above 1.7 with the decimal points in one vertical line, ones under ones and tenths under tenths. In the tenths column 4 plus 7 makes 11, so write 1 and carry 1; in the ones column 2 plus 1 plus the carried 1 makes 4. The answer is 4.1. Writing 3.11 means the 11 was dropped in without carrying. Subtraction works the same way: 5.0 minus 2.6, points aligned, borrow one from the ones to make ten tenths, 10 minus 6 is 4, then 4 minus 2 is 2, so the answer is 2.4. Require the pupil to touch both points with a finger before starting.

Points aligned means tenths under tenths.

04 Question types and worked steps

Type	Example	Steps	Answer
Meaning	How many tenths are in 0.7	Look at the tenths place: the 7 counts tenths	Seven, that is 7/10
Comparing	Which is larger, 0.9 or 1.2	On the line 0.9 is left of 1 and 1.2 is right of it	1.2
Column addition	2.4 plus 1.7	Align points; tenths 4 plus 7 is 11, write 1 carry 1; ones 2 plus 1 plus 1 is 4	4.1
Conversion	Three dollars five cents in dollars	Ten cents make one dollar, so five cents is 0.5 dollars	3.5 dollars

Sanity check (teach this): Sense check: 0.7 plus 0.7 is 1.4, a little more than one, which matches the number line. The sum of 2.4 and 1.7 must lie between 2 and 5, so 4.1 is sensible while 3.11 is nonsense because both numbers exceed one. Three dollars five cents must be less than four dollars, so 3.5 is right and 35 is ten times too big. Forty-seven millimetres is under five centimetres, so 4.7 centimetres is sensible.

05 Independent practice (3 questions)

Question	Answer	What to watch
Addition	0.6 plus 0.7	Tenths 6 plus 7 is 13, write 3 carry 1, so the answer is 1.3
Subtraction	4.0 minus 1.8	Points aligned, borrow to make ten tenths, 10 minus 8 is 2, 3 minus 1 is 2, so 2.2
Conversion	Twenty-six millimetres in centimetres	Ten millimetres make one centimetre, so 2.6 centimetres
Comparing	Which is larger, 2.9 or 3.1	2.9 has not reached 3 while 3.1 has, so 3.1

06 Common mistakes and what to say

Mistake	Student writes	Say this
Points not aligned	Writes 3.11 for 2.4 plus 1.7	The two decimal points must sit above each other. You added 4 and 7 to make 11, but that 11 has to be carried, not written straight after the point.
Tenths added as whole numbers	Answers 0.13 for 0.6 plus 0.7	Six tenths plus seven tenths is thirteen tenths, and ten tenths make one, so the answer is 1.3.
Reading confused with value	Writes 5 for 0.5	0.5 is five tenths. Writing 5 makes it ten times larger.
Conversion the wrong way	Writes 47 centimetres for 47 millimetres	Millimetres are smaller than centimetres, so the number must shrink: 4.7 centimetres.

07 Homework (paper, section and question numbers)

Primary 3 maths paper, decimal application — Section E, question 3: 1.2 L of juice and 450 mL
Primary 3 maths paper, units — Section B, question 6: one litre in millilitres
Primary maths chapter index — All chapters in one table

08 Exit check (last 5 minutes)

Question	Answer	Decision
0.8 plus 0.5	1.3	An answer of 0.13 means no carrying; reteach act three
3.2 minus 0.7	2.5	An answer of 3.5 means borrowing went the wrong way; redo the column
Fifteen cents in dollars	1.5 dollars	An answer of 15 dollars means the tenths relation is missing; redo the coin work

09 Tutor FAQ

Should Primary 3 pupils write decimals at all?

Yes, but only one place, and always introduced through tenths. A pupil who cannot say that 0.7 is seven tenths will treat the digits as two separate whole numbers.

Why insist on aligned columns?

It is the only reliable way to stop the answer 3.11. Once the points are aligned, tenths sit under tenths and the carry has a place to go.

When are two-place decimals taught?

In Primary 4. If a pupil asks about 7.48, just say there are two places after the point and do not set questions on it.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.