Answer
$11210_{\text{three}}$
Work Step by Step
The place values of base three are: $...,3^5, 3^4, 3^3, 3^2, 3^1, 1$.
The place values that are less than or equal to $129$ are $3^4, 3^3$, $3^2$, $3^1$, and $1$
Divide $129$ by $3^4$ or $81$ to obtain:
$129 \div 81 = 1$ remainder $48$
Divide the remainder $48$ by $3^3$ or $27$ to obtain:
$48 \div 27 = 1$ remainder $21$
Divide the remainder $21$ by $3^2$ or $9$ to obtain:
$21 \div 9 = 2$ remainder $3$
Divide the remainder $3$ by $3^1$ or $3$ to obtain:
$3 \div 3 = 1$ remainder $0$
Divide the remainder $0$ by $1$ to obtain:
$0\div 1 = 0$ remainder $0$
Thus,
$129=(1 \times 3^4) + (1 \times 3^3) + (2 \times 3^2) + (1 \times 3^1) + (0 \times 1)
\\129=11210_{\text{three}}$
