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12 votes
12 votes
Decide !!!!!!!!!!!!!!!!!

Decide !!!!!!!!!!!!!!!!!-example-1
User Sargturner
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1 Answer

14 votes
14 votes

Observe that

7¹ ≡ 7 (mod 100)

7² ≡ 49 (mod 100)

7³ ≡ 343 ≡ 43 (mod 100)

and (using Euclid's algorithm)

7 ⁻¹ ≡ 43 (mod 100)

so that

7⁴ ≡ 7³ × 7¹ ≡ 43 × 7¹ ≡ 1 (mod 100)

This means

7⁷ ≡ 7⁴ × 7³ ≡ 1 × 43 ≡ 43 (mod 100)

Next,

(7⁷)⁷ ≡ 43⁷ ≡ (7 ⁻¹)⁷ ≡ (7⁷) ⁻¹ ≡ 43⁻¹ ≡ 7 (mod 100)

(3 7's)

Next,

((7⁷)⁷)⁷ ≡ 7⁷ ≡ 43 (mod 100)

(4 7's)

A pattern emerges: a power tower involving an even number of 7's will reduce to 43 (mod 100), so the last two digits of ((7⁷)⁷ ... )⁷ with 2020 7's will have 43 as its last two digits.

User Jakemingolla
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3.0k points