AASA Grade 4 Operations and Algebraic Thinking. Practice it free.

The sum of the digits of $81$ is $8 + 1 = 9\textsf{,}$ and $9$ is divisible by $9\textsf{,}$ so $81$ is divisible by $9\textsf{.}$

$36$ is a multiple of $9$: $9\textsf{,}$ $18\textsf{,}$ $27\textsf{,}$ $36\textsf{,}$ $\ldots$

$15$ has factors $1\textsf{,}$ $3\textsf{,}$ $5\textsf{,}$ and $15$, so it is composite.

Add the digits: $1 + 1 + 1 = 3\textsf{.}$

Four $8\text{s}$ make $32$: $8\textsf{,}$ $16\textsf{,}$ $24\textsf{,}$ $32\textsf{.}$

$29$.

The last digit of $20$ is $0\textsf{,}$ so $20$ is divisible by $10\textsf{.}$

Five $6\text{s}$ make $30$: $6\textsf{,}$ $12\textsf{,}$ $18\textsf{,}$ $24\textsf{,}$ $30\textsf{.}$