12. The marked price of a product is $p$ \% higher than the cost. When the product is sold at a discount, to avoid a loss, the discount on the selling price (i.e., the percentage reduction) must not exceed $d$ \%. Then $d$ can be expressed in terms of $p$ as $\qquad$
12. $\frac{100 p}{100+p}$.

Let the cost of the item be $a$, then we have
" $(1+p \%) (1-d \%) = a$, solving for $d$ gives $d = \frac{100 p}{100+p}$.