101
Centimetres and decimetres
Textbook Part 3, pp. 56–57
GoalMeet the decimetre: 1 dm = 10 cm; express lengths in dm and cm.
New words
decimetre (dm) · detsimetr (dm)1 dm = 10 cm · 1 dm = 10 cmsmall rug · gilamcha
Explanation
For longer things we use decimetres. 1 dm = 10 cm — just like 1 ten = 10 ones. 27 cm = 2 dm 7 cm. We add measurements directly only when they are in the same unit: 11 dm + 8 dm = 19 dm.
Worked examples
A small rug is 35 cm long: 35 cm = 3 dm 5 cm.
AC = 3 cm, CB = 5 cm → AB = 8 cm (the parts add up to the whole segment).
Class activity
Children cut a 10 cm paper strip (a “decimetre stick”) and use it to measure a book, a desk and the width of the door.
Practice
1
40 cm = ? dm
4 dm
2
2 dm 3 cm = ? cm
23 cm
3
Put the sign: 15 cm + 12 cm ... 31 cm
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4
Put the sign: 1 dm ... 9 cm
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