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Modulo Arithmetic & Congruence · Question 75 of 97

Question 75

MediumOriginal question

What is the least positive xx with 5x≡3(mod8)5x \equiv 3 \pmod{8}?

Options for question 75

Show the answer and explanation
CorrectNot quiteAnswer D

Correct answer: D

Explanation

  1. The inverse of 5 modulo 8 is 5, since 5×5=25≡15 \times 5 = 25 \equiv 1.

  2. Multiply both sides by 5: x≡15≡7x \equiv 15 \equiv 7. Check: 5×7=35=32+35 \times 7 = 35 = 32 + 3.

Worth noting

3 copies the right side; 5×3=15≡75 \times 3 = 15 \equiv 7, not 3.

Concept
A linear congruence
Typical time
90 s
Your time
—
The rule this testsSolving with congruences
Practise this chapter →

Ten questions from Modulo Arithmetic & Congruence, at your own pace.

Practise this chapter →

Ten questions from Modulo Arithmetic & Congruence, at your own pace.