Answer
See below
Work Step by Step
There is 1 branch at the first stage. Thus, $a_1=1$
There are 2 branches at the second stage. Thus, $a_2=2^1$
For the third stage: $a_3=2^2$
For the 4th stage, $a_4=2^3$
For the 5th stage, $a_5=2^4$
For the 6th stage, $a_6=2^5$
