15
Adding (21 + 39)
Textbook: Part 1, lesson 15, pp. 32–33
GoalLearn adding tens and ones separately (21 + 39).
New words
tens · o‘nliklarones · birliklarsum · yig‘indigrouping · guruhlash
Explanation
When adding two-digit numbers we add tens to tens and ones to ones: 32 + 28 = (30 + 20) + (2 + 8) = 50 + 10 = 60. If the ones make 10, we get one more ten.
Worked examples
22 + 18 = (20 + 10) + (2 + 8) = 30 + 10 = 40.
83 + 17 = (80 + 10) + (3 + 7) = 90 + 10 = 100.
Class activity
“Paired sums”: children solve a sum like 23 + 27 and then write the inverse sums: 50 − 27 = 23, 50 − 23 = 27.
Practice
1
31 + 29 = ?
60
2
44 + 16 = ?
60
3
39 kg of apples were picked from one tree and 21 kg from another. How many kg in all?
60
4
Children hung 13 nest boxes in the school garden and 27 in the street. How many in all?
40