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Decimal Detectives: follow the place value

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Decimal points can feel tiny but they change a number's value. Solve short cases and practise putting every digit in its right place.

Ages 9+
5–10 min
decimals
decimal addition
decimal subtraction
Free · no sign-up

How to play Decimal Detectives

Choose a grade that matches the learner's current school work.
Pick one topic from the skill cards and try a question.
Read the immediate feedback, then continue at a level that adjusts to answers.
Reach the mastery target or switch to another skill whenever you are ready.

Tips and strategy

Write a zero as a placeholder so the decimal points line up before adding or subtracting.
Ask the learner to explain one answer in their own words; the reasoning matters as much as speed.

What you'll learn

Decimals describe parts of a whole using place value: tenths, hundredths and beyond. In addition and subtraction, lining up decimal points keeps like values together. In multiplication, estimate first, multiply, then check where the decimal belongs by asking whether the product should be larger or smaller.

Worked example

Write 3.6 as 3.60, then line up the decimal points: 3.60 + 1.75 = 5.35. The zero does not change the value; it makes the hundredths column visible. For multiplication, 0.4 × 0.3 means four tenths of three tenths, or twelve hundredths: 0.12. Estimating first helps: the answer should be much smaller than one, so 1.2 would be too large.

A common sticking point

Treating decimals like whole numbers can put a point in the wrong place. Compare 0.8 and 0.35 by writing 0.80 and 0.35: eight tenths is greater than thirty-five hundredths. When adding, keep tenths under tenths and hundredths under hundredths. When multiplying, count the total decimal places in the factors, then use an estimate to check whether the result is sensible.

Try it away from the screen

Look at prices, measurements or a recipe. Ask which amount is greater, then add two prices by writing the decimal points directly under one another. Estimate the total first so the exact answer has a reference. If a child is ready, compare 0.5 with 1/2 and 50%; switching forms shows that decimals are another way to describe parts of a whole.

Mini challenge with answers

Calculate 2.40 + 0.65, then estimate 0.2 × 0.4 before working it out. Answers: 3.05; the product is less than 0.2 and equals 0.08 (two tenths multiplied by four tenths is eight hundredths). Ask which place-value columns matter in the addition and how the estimate helps locate the decimal in the product.

How to use this practice path

For a first session, use addition and subtraction only, because lining up place values is the key idea. Next, compare decimal values written with different numbers of digits, such as 0.6 and 0.58. Introduce multiplication after a learner can estimate whether the product should be smaller or larger than a factor. Finish with an explanation of why a placeholder zero can help without changing a number's value.

Where the idea goes next

Decimals connect naturally to fractions and percentages. A learner can place 0.25, 1/4 and 25% beside the same shaded square, then explain why all three names describe an equal part. This is especially helpful before percentage work, where moving between forms makes estimates easier. Keep the visual model nearby until the equivalence is understood rather than relying only on a rule about moving a decimal point.

This free browser practice has grades 4 to 5 and is intended for learners aged about 9 to 11. Each topic has short questions, a mastery meter and progress saved in this browser on this device. It is independent practice, not a school-board test or a substitute for a teacher's advice about a particular learner.

Want paper practice too? Browse the printable maths worksheets, then explore all Grade 1 to 8 maths skills or ask a parent or guardian to enquire about a class.

Printable worksheets

Fractions as decimals worksheets (Grade 4)
Dividing decimals worksheets (Grade 6)
Practise fractions as decimals online
Today's maths challenge

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FAQ

Decimal Detectives: questions

Is Decimal Detectives free to play?

Yes. It is free, runs in a modern browser and does not need an account or download.

What does Decimal Detectives teach?

Can I choose a grade?

Grades 4 to 5 are available. School grade systems differ, so choose the level that best matches what the learner is studying now.

Does the game save progress?

The game saves mastery scores in local browser storage on this device. It does not create an online student account.

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