Question

Please help!! I don’t know how to solve this. Which one is the digit of the units in 3^81:

A) 3
B) 9
C) 7
D) 1
E) None of the above

Answer 1

• 3⁸¹ = 3⁹ ⁺ ⁹ ⁺ ⁹ ⁺ ⁹ ⁺ ⁹ ⁺ ⁹ ⁺ ⁹ ⁺ ⁹ ⁺ ⁹ ⁺ ⁹
• 3⁸¹ = 3⁹ x 3⁹ x 3⁹ x 3⁹ x 3⁹ x 3⁹ x 3⁹ x 3⁹ x 3⁹
• 3⁹ = 19683 [Using Calculator]

This means that the Unit digit of 19683 is 3. For each number, multiply the unit's digit of 3⁹.

• 3 x 3 x 3 x 3 x 3 x 3 x 3 x 3 x 3
• => 19683
• => Unit digit of 3⁸¹ = 3

Option A is correct.