25
Real-life problems
Textbook: Part 1, lesson 25, pp. 78–80
GoalSolve real-life problems on average mass, tolerance (±) and double inequalities.
New words
average mass · o‘rtacha massadouble inequality · qo‘sh tengsizliklimit (bound) · chegarabag (sack) · xalta
Explanation
The average mass of equal items = total mass : number of items. A label m = 2 000 ± 30 g means the mass may be anything from 2 000 − 30 = 1 970 g to 2 000 + 30 = 2 030 g. We write this as a double inequality: 1 970 ≤ m ≤ 2 030. The sign “≤” means “less than or equal to”.
Worked examples
5 apples weigh 750 g: the average mass is 750 : 5 = 150 g.
If y = 60 ± 8, then 60 − 8 ≤ y ≤ 60 + 8, that is 52 ≤ y ≤ 68.
Class activity
“Scales”: the class weighs several identical things (for example 6 pencils) and works out the average mass of one.
Practice
1
5 pears weigh 900 g. What is the average mass of one pear in g?
180
2
A pack is labelled m = 500 ± 15 g. What is the smallest possible mass in g?
485
3
Write a = 70 ± 4 as a double inequality.
66 ≤ a ≤ 74
4
A shop has 4 sacks of rice of 25 kg and 9 sacks of 3 kg. How many kg of rice is there in total?
127