Primary 6 Maths | Percentages (III): Simple Interest Lesson-Prep Guide (for tutors)

Primary 6 Maths | Percentages (III): Simple Interest Lesson-Prep Guide (for tutors) | Principal, annual rate, time, amount

2026-11-15

Tutor lesson-prep guides · Primary Maths

What this lesson really teaches is not a formula but the phrase annual rate: how much every hundred dollars earns in one year. When the decimal point goes wrong, it is almost always a missing division by a hundred, or months treated as years.

Objectives — The pupil can: (1) explain that an annual rate of 3% means 3 dollars a year on every 100 dollars; (2) compute interest as principal times rate times time, with time in years; (3) tell interest apart from the total amount; (4) work back from interest to the principal or the time.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Percent to decimal	turns 25% into 0.25	moves the point two places
Multiply and divide decimals	works out 8000 times 0.025	gets 200
Time units	knows 6 months is half a year	says 0.5 years

02 Lesson flow (40 minutes)

Time	Stage	Tutor does	Student does
0–6 min	Diagnose the rate	asks what 3% means	says 3 dollars per 100 per year
6–16 min	Where the formula comes from	works 100 dollars over three years	says 3 plus 3 plus 3
16–26 min	Align the three quantities	shows a unit-error line	points out where it breaks
26–34 min	Work back to the principal	reverses from the interest	writes 630 divided by 0.03 times 2
34–40 min	Exit check	three questions	all correct

03 Teaching script (read it out)

Scene 1: Act 1: The rate means per hundred per year

Write annual rate 3% on the board, then write 100 dollars under it. Ask how much 100 dollars earns in one year: 3 dollars. Change to 200 dollars: 6 dollars. Change to 1000 dollars: 30 dollars. Then 8000 dollars: 8000 times 0.03, which is 240 dollars. This step turns the percent sign into a sentence the pupil can say out loud, and only then does the formula mean anything.

Tutor line: “3% is not a magic symbol. You put 100 dollars in the bank and after a year the bank gives you 3 more dollars. That is what 3% is.”

Scene 2: Act 2: Aligning the units, the most common misconception

The usual error is an annual rate with a time given in months. Write: principal 12000 dollars, annual rate 4%, six months. A pupil writes 12000 times 0.04 times 6 and gets 2880, which is wrong, because 6 is months not years. The correct line is 12000 times 0.04 times 0.5, which is 240 dollars. Put both lines side by side and ask which one is plausible: half a year should be about half of a year of interest, not six times it.

Tutor line: “Look at 2880. Half a year earning 2880 is more than a quarter of what you put in. No bank would do that. So the formula is fine; the 6 is the problem.”

Scene 3: Act 3: Working back to the principal

Give interest 630 dollars, annual rate 3%, time 2 years, find the principal. Take it in two steps: interest per year is 630 divided by 2, which is 315 dollars; then 315 divided by 0.03, which is 10500 dollars. Two short lines beat one long expression. Always check by substitution: 10500 times 0.03 times 2 equals 630.

Tutor line: “Ask yourself first: 630 over two years, so how much in one year? Once you have one year, you can find the principal.”

04 Question types and worked steps

Type	Example	Steps
Find the interest: principal, rate and time all given	convert to a decimal, multiply the three, write the unit	8000 times 0.025 times 3 equals 600 dollars
Find the total amount: interest plus principal	interest first, then add the principal	8000 plus 600 equals 8600 dollars
Time given in months: convert to years first	months divided by 12, then into the formula	12000 times 0.04 times 0.5 equals 240 dollars
Work back to the principal: interest, rate and time known	interest divided by time, then by the rate, then check	630 divided by 2 is 315; 315 divided by 0.03 is 10500 dollars

Sanity check (teach this): Primary 6 handles simple interest only, not compound interest. Time is always in years; months are divided by 12 first. Answers carry the dollar unit, and interest and total amount are written separately. To check, substitute the principal or time back into the original line and see whether the interest comes out right.

05 Independent practice (3 questions)

Question	Answer	What to watch
Principal 8000 dollars, annual rate 2.5%, three years: how much interest?	600 dollars	did the pupil divide by 100
For the same account, what is the total after three years?	8600 dollars	was the principal added back
Principal 12000 dollars, annual rate 4%, six months: how much interest?	240 dollars	were the six months turned into 0.5 years
Interest 630 dollars, annual rate 3%, two years: what is the principal?	10500 dollars	was the interest divided by the time first
6000 dollars for two years at 5%, or 5000 dollars for two years at 6%: which earns more?	the same, 600 dollars each	is the pupil willing to compute both sides

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating the rate as a whole number	8000 times 2.5 times 3 equals 60000	“2.5% is 0.025, not 2.5. The percent sign itself means divide by a hundred.”
Months used as years	12000 times 0.04 times 6 equals 2880	“Half a year should be about half of a full year. Check whether 2880 makes sense.”
Confusing interest with the total amount	answering 600 when the question asks for the total	“The question says in total, so that is the principal plus the interest. Add the 8000 back.”
Forgetting the time when reversing	630 divided by 0.03 equals 21000	“The 630 is the total for two years. Divide by 2 for one year first, then by the rate.”

07 Homework (paper, section and question numbers)

Primary 6 maths paper 1, percentages section — Section C, questions 1 to 3
Primary maths lesson-prep chapter index — Percentages chapters I to III

08 Exit check (last 5 minutes)

Question	Answer	Decision
Principal 2000 dollars, annual rate 5%, four years: how much interest?	400 dollars, total 2400 dollars	if wrong: reteach Act 1
Interest 180 dollars, annual rate 3%, two years: what is the principal?	3000 dollars	if wrong: reteach Act 3
Interest 600 dollars, principal 6000 dollars, annual rate 5%: for how many years?	2 years	if wrong: reteach reversing the time

09 Tutor FAQ

Why teach interest in Primary 6?

Interest is the most common real setting for percentages, and it is the base for secondary algebra and for everyday money sense. The simple-interest idea is enough; compound interest waits for secondary school.

The pupil knows the formula but fails as soon as the numbers change. What helps?

That means letters were memorised instead of a relationship. Go back to 100 dollars for one year, then 200 and 1000, and let the pupil say it three times before writing the formula.

Should compound interest be included?

Not in this chapter. Primary 6 stays with simple interest, where the yearly interest never earns interest itself.

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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.

Practice papers　Paper 1 (basics)　·　Paper 2 (intermediate)　·　Paper 3 (challenge)　·　Paper 4 (error clinic)