Logic circuits

How many rows does the truth table of a 3-input logic circuit have?

  1. A6
  2. B9
  3. C8
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Answer: C. 8

Each input can be 0 or 1, so three inputs have 2³ = 8 combinations. A truth table lists each input combination on its own row, so it has 8 rows. With n inputs there are 2ⁿ rows.

  • A. 6 is 3 × 2, which is not how combinations are counted.
  • B. 9 is 3², not the number of combinations of binary inputs.
  • C. Correct. 2³ = 8 rows.
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