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WAEC Mathematics 2020 Question 4

Question 4

Modular Arithmetic

Find the least value of x which satisfies the equation 4x = 7 (mod 9)

  1. A.7
  2. B.6
  3. C.5
  4. D.4
Show the answer and explanation

Answer: D (4)

Explanation

Try values: 4 × 4 = 16 = 9 + 7, so 16 ≡ 7 (mod 9). Smaller values 1, 2, 3 give 4, 8, 3, so x = 4 is the least.