67
A way to find area
Textbook: Part 2, lesson 67, pp. 87–89
GoalA way to find area: counting units and parts.
New words
unit of measure · o‘lchov birligimung bean · mosh donihalf · yarmiquarter · chorak
Explanation
Area can be found by choosing a unit of measure and counting. If 6 mung beans fit in a 1 cm² square and a shape holds 39 beans, then 39 : 6 = 6 remainder 3, so the area is a little more than 6 cm². A half, a third and a quarter of a shape are one of 2, 3 or 4 equal parts of it. For a sum with a product: 15 one-litre and 18 three-litre jars hold 15 + 18 × 3 = 69 l.
Worked examples
39 beans, 6 per cm²: 6 full cm² and 3 beans more.
A 6 cm × 4 cm rectangle has area 24 cm²; half is 12 cm² and a quarter is 6 cm².
Class activity
“Area with rice”: count how many grains of rice or beans fit in a 1 cm² square on squared paper and estimate the area of the outline of a hand.
Practice
1
6 mung beans fit in a 1 cm² square and 39 beans fit in a shape. How many full cm² does the shape contain?
6
2
A shop has juice in 15 one-litre and 18 three-litre jars. How many litres of juice are there in total?
69
3
Half of a 6 cm by 4 cm rectangle is painted. How many cm² are painted?
12
4
If one third of the same rectangle is painted, how many cm² are painted?
8