3
Hundreds in the sum
Discover Mathematics P2: Term 2, Lessons 81–90
GoalAdd numbers when the sum reaches a hundred or more, including three-digit numbers.
New words
hundreds · groups of 100, the third placecarry · move the regrouped ten or hundred to the next columnthree-digit number · a number with hundreds, tens and ones
Explanation
Sometimes ten tens appear when we add. Ten tens make one hundred, so regroup again. In 76 + 28, the ones are 6 + 8 = 14: write 4 and carry 1 ten. The tens are 7 + 2 + 1 = 10 tens, which is 1 hundred and 0 tens. The sum is 104. For three-digit numbers we add ones, then tens, then hundreds, and carry when a column reaches 10 or more. In 377 + 235, ones 7 + 5 = 12, tens 7 + 3 + 1 = 11, hundreds 3 + 2 + 1 = 6. The sum is 612.
Worked examples
76 + 28: ones 14, carry 1. Tens 7 + 2 + 1 = 10, which is 1 hundred. The sum is 104.
146 + 154: ones 6 + 4 = 10, tens 4 + 5 + 1 = 10, hundreds 1 + 1 + 1 = 3. The sum is 300.
377 + 235: ones 12, tens 11, hundreds 6. The sum is 612.
Class activity
Big number race. Take two sets of digit cards from 0 to 9. Each player makes a three-digit number, then both numbers are added. The player with the greater sum scores a point. Estimate first by rounding, and then add exactly.
Practice
1
Add 87 + 65.
7 + 5 = 12, carry 1. 8 + 6 + 1 = 15 tens. The sum is 152.
2
Add 99 + 8.
9 + 8 = 17, carry 1. 9 + 1 = 10 tens. The sum is 107.
3
Add 250 + 175.
0 + 5 = 5, 5 + 7 = 12 tens (carry 1 hundred), 2 + 1 + 1 = 4. The sum is 425.
4
Add 346 + 255.
6 + 5 = 11, 4 + 5 + 1 = 10, 3 + 2 + 1 = 6. The sum is 601.
5
A library has 129 story books and 134 science books. How many books in all?
129 + 134 = 263 books.