Compute the binary multiplication: 111 (base 2) × 101 (base 2). What is the product in binary?

Difficulty: Easy

Correct Answer: 100011 (base 2)

Explanation:


Introduction / Context:
Binary multiplication is essential for digital arithmetic units. The operation mirrors decimal long multiplication but uses shifts and additions in base 2. Here we multiply 111₂ by 101₂ and provide the binary product.


Given Data / Assumptions:

  • Multiplicand = 111 (base 2) = 7 decimal.
  • Multiplier = 101 (base 2) = 5 decimal.
  • We perform exact multiplication and present the result in binary.


Concept / Approach:
One approach is to convert to decimal (7×5=35) and convert back to binary (35 = 32+2+1 = 100011₂). Alternatively, perform binary long multiplication using partial products and left shifts corresponding to the multiplier’s 1-bits.


Step-by-Step Solution:

Partial products with multiplier 101₂ (bits from LSB to MSB): Bit 0 = 1 → add 111₂ shifted by 0 → 000111₂. Bit 1 = 0 → add 000₂ (no contribution) shifted by 1 → 000000₂. Bit 2 = 1 → add 111₂ shifted by 2 → 11100₂. Sum partials: 11100₂ + 000111₂ = 100011₂.


Verification / Alternative check:
Decimal cross-check: 111₂ = 7 and 101₂ = 5; 7×5=35. Convert 35 to binary: 35 = 32 + 2 + 1 → 100011₂, matching the computed result precisely.


Why Other Options Are Wrong:

110011₂ (51 decimal) and 111100₂ (60 decimal) do not equal 35. 000101₂ equals 5 decimal, which is just the multiplier, not the product. “None” is invalid because 100011₂ is correct.


Common Pitfalls:
Misaligning shifts when forming partial products and forgetting that a 0 bit produces no partial product. Always add carefully and confirm by converting to decimal if in doubt.


Final Answer:
100011 (base 2)

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