141
Multiplying and dividing by 10 and 100
Textbook: Part 4, lesson 131, pp. 34–35
GoalUse multiplying and dividing by 10 and 100 in bracket expressions, equations and problems.
New words
round number · yaxlit sonbracket · qavsequation · tenglamaarea · yuz (maydon)
Explanation
First calculate inside the brackets, then multiply by 10 or 100: (13 + 38) × 10 = 51 × 10 = 510. In an equation use the opposite operation: z × 100 = 400, z = 400 : 100 = 4. If n : 10 = 85, then n = 85 × 10 = 850. In a problem, “10 times smaller” means divide by 10.
Worked examples
(91 − 27) × 100 = 64 × 100 = 6400.
700 : m = 7, m = 700 : 7 = 100.
Class activity
“Relay”: in a 10-stage relay each stage gives 10 points. Teams complete stages, express their points as “10 × …” and show when they reach 100.
Practice
1
(13 + 38) × 10 = ?
510
2
(91 − 27) × 100 = ?
6400
3
42 × x = 420, x = ?
10
4
n : 10 = 85, n = ?
850
5
One school sports ground has an area of 250 m². A second one is 10 times smaller. What is the total area of both grounds in m²?
275