Answer:
811
Step-by-step explanation:
The place values of digits in base-3 are all powers of 3. The numbers 1, 3, 4, 9 written in base-3 are ...
1
10
11
100
You will recognize these as successive numbers in base 2. Then the 81st number of interest is written in base-3 the way 81 is written in base-2:
101 0001
It will be the sum 3^6 +3^4 +3^0 = 729 +81 +1 = 811
The 81st number is 811.
