1
Two-digit sums with a carry
Discover Mathematics P2: Term 2, Lessons 81–90
GoalAdd two-digit numbers with carrying when the sum passes 100, and draw place value pictures.
New words
place value picture · a drawing of hundreds, tens and onescarry · move 10 ones to the tens, or 10 tens to the hundredssum · the answer to an addition
Explanation
We can draw a flat square for 100, a long rod for 10 and a small dot for 1. To add 59 + 48, draw the 5 rods and 9 dots, and the 4 rods and 8 dots. The ones make 9 + 8 = 17, which is 1 ten and 7 ones. The tens make 5 + 4 + 1 = 10, which is 1 hundred and 0 tens. So the sum is 1 hundred, 0 tens and 7 ones: 107. Two carries happened, one from the ones and one from the tens. Always add the ones first, then the tens, then the hundreds.
Worked examples
59 + 48: ones 9 + 8 = 17, tens 5 + 4 + 1 = 10. The sum is 107, shown in the picture.
86 + 37: ones 6 + 7 = 13, tens 8 + 3 + 1 = 12. That is 1 hundred, 2 tens and 3 ones: 123.
168 + 25: ones 8 + 5 = 13, tens 6 + 2 + 1 = 9, hundreds 1. The sum is 193.
Class activity
Draw and add. Take a sheet of squared paper. Draw squares for hundreds, lines for tens and dots for ones to show 67 and 58. Count the ones and trade 10 dots for a line. Then count the tens and trade 10 lines for a square. Write the sum.
Practice
1
Add 67 + 58.
7 + 8 = 15, 6 + 5 + 1 = 12. The sum is 125.
2
Add 95 + 27.
5 + 7 = 12, 9 + 2 + 1 = 12. The sum is 122.
3
Add 148 + 36.
8 + 6 = 14, 4 + 3 + 1 = 8, hundreds 1. The sum is 184.
4
Add 79 + 79. It is a double.
9 + 9 = 18, 7 + 7 + 1 = 15. The sum is 158.
5
Add 205 + 98.
5 + 8 = 13, 0 + 9 + 1 = 10, 2 + 1 = 3. The sum is 303.