$\frac{1}{15},\frac{n}{30}$ is interpreted as:

$1\%$ cash discount if the whole amount is paid within $15$ days, the balance is due in $30$ days.

$15\%$ cash discount if the whole amount is paid within $1$ day, the balance is due in $30$ days.

$30\%$ discount if paid within $15$ days.

$1\%$ discount if paid within $30$ days.