Question

Decide !!!!!!!!!!!!!!!!!

Answer 1

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.