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Modulo Arithmetic & Congruence · Question 71 of 97

Question 71

MediumOriginal question

Chairs set out in rows of 3 leave 2 over, and in rows of 5 leave 3 over. What is the least number of chairs?

Options for question 71

Show the answer and explanation
CorrectNot quiteAnswer B

Correct answer: B

Explanation

  1. We need x≡2(mod3)x \equiv 2 \pmod{3} and x≡3(mod5)x \equiv 3 \pmod{5}.

  2. Numbers ≡3(mod5)\equiv 3 \pmod{5}: 3, 8, 13, … The first that leaves 2 on division by 3 is 8.

Worth noting

23 works too but is not the least; solutions repeat every 15.

Concept
Two congruences together
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.