Track / Stage 4 / 259 of 340 #519 of 1964

Problem 519

AMC 12 late, AIME early

Number theory Difficulty 4.9 Find the answer HMMT February

A sequence is defined by $a_{0}=1$ and $a_{n}=2^{a_{n-1}}$ for $n \geq 1$ . What is the last digit (in base 10) of $a_{15}$ ?

Your answer

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Official solution

6. Certainly $a_{13} \geq 2$ , so $a_{14}$ is divisible by $2^{2}=4$ . Writing $a_{14}=4 k$ , we have $a_{15}=2^{4 k}=16^{k}$ . But every power of 16 ends in 6, so this is the answer.

Source: Omni-MATH, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.