133
Order of operations
Textbook: Part 4, lesson 124, pp. 18–19
GoalDo the operations in expressions with brackets, starting inside the brackets.
New words
bracket · qavsinside the brackets · qavs ichiorder · tartibmistake · xato
Explanation
In an expression with brackets: 1) first the operations inside the brackets, 2) then multiplication and division, 3) finally addition and subtraction. (81 − 9) : 9 = 72 : 9 = 8, but 81 − 9 : 9 = 81 − 1 = 80. Brackets change the answer! 6 + 8 : 2 = 10 is wrong for “half of the sum of 6 and 8”; we write (6 + 8) : 2 = 7.
Worked examples
100 − (91 − 9) = 100 − 82 = 18.
(12 − 3) × 4 = 9 × 4 = 36, but 12 − 3 × 4 = 12 − 12 = 0.
Class activity
“Bracket magic”: write two expressions with the same numbers: 45 − 25 : 5 and (45 − 25) : 5. Children find both values and discuss how the brackets changed the result.
Practice
1
254 − (60 + 6) = ?
188
2
(45 − 25) : 5 = ?
4
3
45 − 25 : 5 = ?
40
4
(12 − 3) × 4 = ?
36
5
Aunt Halima bought 30 m of cloth. She made a sheet from 18 m and 6 equal pillowcases from the rest. How many metres of cloth did one pillowcase take?
2