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Practical tasks
Textbook: Part 1, lesson 40, pp. 86–87
GoalOperations with zero; a − b − c = a − (b + c); solving a problem two ways.
New words
zero · nolexpression · ifodatwo ways · ikki usulexhibit · eksponat
Explanation
Adding zero to a number or subtracting zero from it does not change it: a + 0 = a, a − 0 = a. A number minus itself is zero: a − a = 0. For subtracting twice: a − b − c = a − (b + c). When a problem is solved in two ways the answers must match.
Worked examples
49 − 26 − 7 = 49 − (26 + 7) = 49 − 33 = 16.
18 + y = 18 → y = 0; 34 − m = 0 → m = 34.
Class activity
“Magic zero”: children make sums with zero (a + 0, a − 0, a − a), turn them into equations and give them to a friend to solve.
Practice
1
x − 25 = 25. x = ?
50
2
34 − m = 0. m = ?
34
3
31 − 12 − 15 = ?
4
4
A museum has 80 exhibits: 30 on the first floor and 25 on the second. How many are on the third floor?
25