Answer
$1001_{\text{two}}$
Work Step by Step
The place values of base two are: $,,,, 2^3, 2^2, 2^1, 1$.
The place value that are less than or equal to $9$ are $2^3, 2^2, 2^1$ and $1$
Divide $9$ by $2^3$ or $8$ to obtain:
$9 \div 8 = 1$ remainder $1$
Divide the remainder $1$ by $2^2$ or $4$ to obtain:
$1 \div 4 = 0$ remainder $1$
Divide the remainder $1$ by $2^1$ or $2$ to obtain
$1 \div 2= 0$ remainder $1$
Divide the remainder $1$ by $1$ to obtain:
$1 \div 1 = $1$ remainder $0$
Thus,
$9=(1 \times 2^3) + (0 \times 2^2) + (0 \times 2) + (1 \times 1)
\\8=1001_{\text{two}}$
