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Chapter 26 · Decimal and Binary Numeration

Lesson 26.3 — Add and Subtract with Binary Carries and Borrows

Align equal place values, exchange two lower-place units for one higher-place unit, and reverse that exchange when a subtraction column needs a borrow.

10 interactive missionsBinary addition and subtractionCarries and borrow chainsOffline core lesson
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One exchange rule powers both algorithms

2 × 2k = 1 × 2k+1
two units in one place1 + 12 × 2k
one unit in the next place1021 × 2k+1

Addition bundles two units and carries one place left. Subtraction unbundles one higher-place unit into two lower-place units.

1
Place-value alignment

Put equal powers of two in the same column

Not complete

Binary digits must be aligned by place value. Added leading zeros are placeholders; they do not change a numeral's value.

Alignment checkpoint.
Need a hint?

Align the ones columns on the right.

2
One-column addition

Write one bit and carry the rest

Not complete
0 + 0 = 0write 0, carry 0
0 + 1 = 1write 1, carry 0
1 + 1 = 10₂write 0, carry 1
1 + 1 + 1 = 11₂write 1, carry 1

Mini-adder

write in this column0
carry to the left1

All one-column states

Carry checkpoint.
Need a hint?

Divide the column total by 2: its remainder is the written bit.

3
original foundation addition

Follow every carry in 101₂ + 111₂

Not complete
decimal check5 + 7
decimal total12
binary result1100₂
carry columns3

Inspect one column at a time

Rebuild the worked example foundation calculation.
Need a hint?

Include the incoming carry before finding the new bit.

4
Worked example 1 and Guided Practice 1

Use the column algorithm on longer additions

Not complete
first value22
second value10
total32
carry columns4
Worked example and practice.
Need a hint?

Pad the shorter addend with leading zeros.

5
Added carry-chain laboratory

Watch one carry trigger the next

Not complete

A carry can create a new total of two in the next column. This is a carry chain.

Carry-chain report

Columns that send a carry: 4

Longest consecutive carry chain: 4

Carry-chain checkpoint.

Count a column only if it sends a carry to its left. Do not count the final new column that merely receives the last carry. These questions use the printed numerals, independent of the explorer settings.

Need a hint?

A carry continues whenever it meets another 1.

6
One-column subtraction

Borrow one higher-place unit as two lower-place units

Not complete
Reverse the exchange: one unit of 2k+1 becomes two units of 2k. Binary borrowing is “borrow one, receive two here.”

Mini-subtractor

write in this column1
borrow from the left1

Borrow chain picture

borrowed higher unit1 × 2k+1
available lower units10₂ × 2k

If a zero column must pass a borrow through, it borrows from farther left and then passes one unit right.

Borrow checkpoint.

For the last two questions, the current column is 0−1 with no borrow already owed. State the written bit and the new borrow flag separately.

Need a hint?

One higher-place unit is worth two units here.

7
original foundation subtraction

Pass a borrow through zero columns

Not complete

The row labeled “borrow in flags” records whether each column owes one unit to the column on its right. These are borrow flags. Subtract that owed unit before subtracting the bottom bit. A zero column may need to borrow from farther left to pay what it owes.

decimal check12 − 3
decimal result9
binary result1001₂
borrow columns2

Inspect one column at a time

Rebuild the worked example foundation subtraction.
Need a hint?

A zero column that owes a borrow must borrow again from the left.

8
Worked example 2, Guided Practice 2, and Test 26

Audit any nonnegative binary subtraction

Not complete
top value109
bottom value94
difference15
borrow columns4
original and test results.

Give each subtraction result in binary. The final choice compares numerical values, not the lengths of the written numerals.

Need a hint?

Subtract the bottom bit and the borrow owed to the right.

9
Independent practice

Binary carry-and-borrow workshop

Not complete
Optional practice: five more calculations

All five answers are binary. In the final expression, do the addition first and then the subtraction. Within each column calculation, work from the rightmost place toward the left.

Show a hint

Align equal places, then check by converting the inputs and result to decimal.

Show a hint

Align equal places, then check by converting the inputs and result to decimal.

Show a hint

Align equal places, then check by converting the inputs and result to decimal.

Show a hint

Align equal places, then check by converting the inputs and result to decimal.

Show a hint

Align equal places, then check by converting the inputs and result to decimal.

Correct all eight questions to complete the workshop. Write calculation results in binary; write counts and requested decimal values in decimal.

Need a hint?

Answers: 10001₂; 100100₂; 1111₂; 11111₂; 5; 35; 11000₂; 2. Decimal checks: 11+6=17; 29+7=36; 16−1=15; 42−11=31; 37−13=24. Five consecutive 1s each send a carry when adding 1.

10
Objective exit ticket

Prove that you can carry, borrow, and verify

Not complete
Need a hint?

Answers: 100011₂; 1001₂; write 1/carry 1; 1010101₂; 32. Decimal checks are 26+9=35, 12−3=9, 106−21=85, and 22+10=32.

10₂

Binary Carry & Borrow Navigator

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Binary Learner

aligned powers of two, completed binary additions and subtractions, explained carry and borrow chains, and verified results in decimal.