2
Ten ones make a new ten
Discover Mathematics P2: Term 1, Lessons 31–40
GoalAdd two 2-digit numbers when the ones make 10 or more by trading 10 ones for 1 ten.
New words
regroup · trade 10 ones for 1 tenones · single cubes, the small part of a numbertens · sticks of ten, the big part of a numbercarry · the new ten that moves to the tens place
Explanation
Add 37 + 26. First add the ones: 7 + 6 = 13. Thirteen ones are too many for the ones place. Ten ones make a new ten, so trade them for one ten stick. That leaves 3 ones. Now add the tens: 3 + 2 and the new ten makes 3 + 2 + 1 = 6 tens. So 37 + 26 = 63.
Worked examples
48 + 25: the ones are 8 + 5 = 13, which is 1 ten and 3 ones. The tens are 4 + 2 + 1 = 7. So 48 + 25 = 73. The picture shows all the pieces together: 6 tens and 13 ones.
56 + 27: the ones 6 + 7 = 13, so write 3 ones and make 1 new ten. The tens 5 + 2 + 1 = 8. So 56 + 27 = 83.
24 + 36: the ones 4 + 6 = 10. Ten ones make exactly 1 new ten and 0 ones are left. The tens 2 + 3 + 1 = 6. So 24 + 36 = 60.
Class activity
Trade game. Use 40 small beans or pebbles. The child makes a number such as 27 with tens (a line of 10 beans) and ones (single beans). Add 15 more beans. Count the loose ones, and every time there are 10 loose beans, trade them for a new line of ten. Say the new number aloud.
Practice
1
Add 26 + 19. The picture shows all the pieces.
Ones 6 + 9 = 15, tens 2 + 1 + 1 = 4. The sum is 45.
2
Add 38 + 45.
Ones 8 + 5 = 13, tens 3 + 4 + 1 = 8. The sum is 83.
3
Seif has 17 stickers. Hana gives him 25 more. How many stickers does he have now?
17 + 25 = 42 stickers.
4
The picture shows 4 tens and 16 ones. Trade ten ones for a new ten. How many are there in all?
16 ones = 1 ten and 6 ones, so 5 tens and 6 ones. The number is 56.
5
Marwan added 45 + 27 and wrote 612. What did he forget? What is the right sum?
He forgot to regroup. 12 ones are 1 ten and 2 ones. The right sum is 72.