36
Subtracting a number from a sum
Textbook: Part 1, lesson 35, pp. 74–75
GoalPractise subtracting a number from a sum; finding the value of a letter expression.
New words
subtracting from a sum · yig‘indidan ayirishconvenient way · qulay usulvalue · qiymatcomparing · taqqoslash
Explanation
(a + b) − c can be calculated in two ways: (a − c) + b or a + (b − c). We choose whichever is easier, usually the addend that is at least c. We replace letters by numbers to find the value of an expression.
Worked examples
(25 + 24) − 14 = 25 + (24 − 14) = 25 + 10 = 35.
a = 49, b = 15, c = 19: (a + b) − c = (49 − 19) + 15 = 30 + 15 = 45.
Class activity
“Two roads”: children solve one sum in two different ways. The class votes on which way was quicker.
Practice
1
(45 + 46) − 25 = ?
66
2
(28 + 16) − 6 = ?
38
3
(67 + 26) − 6 = ?
87
4
On a farm 40 eggs were collected in the morning and 35 in the evening. 15 were kept and the rest sent to a shop. How many eggs were sent?
60