Place Value, Decimals and Rounding

Reading and ordering decimals, multiplying and dividing by powers of ten, and rounding well enough that an estimate can be trusted to catch a wrong answer.

หลักสูตรแกนกลาง / สสวท.: จำนวนและพีชคณิต (Number and Algebra)ตรงกับ ม.1 บทที่ 4 ทศนิยมและเศษส่วน (ส่วนทศนิยม)
Common Core (US): 5.NBT, 6.NS
Cambridge: Lower Secondary Stage 7 — Number: place value, ordering and rounding

3.1 Decimal place value

Column	Value	As a fraction
hundreds	$100$	—
tens	$10$	—
ones	$1$	—
tenths	$0.1$	$\frac{1}{10}$
hundredths	$0.01$	$\frac{1}{100}$
thousandths	$0.001$	$\frac{1}{1000}$

What each column is worth

Comparing two decimals

1Compare the whole-number parts first
2If they match, compare tenths, then hundredths, and so on
3Pad the shorter number with zeros so the columns line up

ข้อควรระวัง

$0.7$ is greater than $0.68$ , even though $68$ looks bigger than $7$ . Compare column by column, not by counting digits — this is the single most common decimal error.

ตัวอย่าง

Put in order, smallest first: $0.4$ , $0.39$ , $0.421$ , $0.4$

วิธีทำ

1.Pad to three decimal places: $0.400$ , $0.390$ , $0.421$ , $0.400$
2.Now compare as if they were whole numbers: $390 < 400 = 400 < 421$

ตอบ $0.39, 0.4, 0.4, 0.421$

📖 2 more sections of this chapter are being checked

The whole chapter is written; we read it back section by section before releasing it, because maths nobody has checked should not be on a public page. What is open is complete — nothing here is abridged.

Read it — now practise it

Generate a worksheet on this topic, or sit a timed mock paper.

Build a worksheet Practice arena