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Modulo Arithmetic & Congruence · Question 72 of 97

Question 72

MediumOriginal question

What is the multiplicative inverse of 7 modulo 10?

3

Correct answerYour answer

4

Correct answerYour answer

7

Correct answerYour answer

9

Correct answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer A

Correct answer: A

We need $7x \equiv 1 \pmod{10}$ : a multiple of 7 ending in 1.
$7 \times 3 = 21$ , so the inverse is 3.

Worth noting

9 gives $7 \times 9 = 63 \equiv 3$ , not 1.

Concept: The multiplicative inverse modulo n
Typical timeTypical: 90 s
Your timeYours: —

The rule this testsSolving with congruences

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