104
Dividing by 3
Textbook: Part 3, lesson 98, pp. 56–57
GoalDivide by 3; tell sharing equally from splitting into parts of a given size.
New words
dividing by 3 · 3 ga bo‘lishequal parts · teng qismlarsegment · kesmainverse operation · teskari amal
Explanation
12 : 3 = 4, because 4 × 3 = 12. Division has two meanings. If we cut a 6 cm segment into 3 equal parts, each is 2 cm. If we cut a 6 cm segment into 3 cm pieces, we get 2 pieces. In both cases 6 : 3 = 2, but the 2 means something different: the length of a part in one, the number of parts in the other.
Worked examples
From 4 × 3 = 12 we get four equalities: 3 × 4 = 12, 12 : 3 = 4 and 12 : 4 = 3.
An equilateral triangle with perimeter 15 cm has side 15 : 3 = 5 cm.
Class activity
“Two kinds of division”: children cut a 12 cm string, first into 3 equal parts and then into 3 cm pieces, and compare the two results.
Practice
1
Three pupils watered 12 trees, sharing equally. How many trees did each water?
4
2
An equilateral triangle has perimeter 15 cm. How many cm is its side?
5
3
27 : 3 = ?
9
4
A 12 m beam was cut into pieces of 3 m each. How many pieces were made?
4
5
Put a sign (<, > or =): 18 : 3 and 24 : 3
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