If Heidi can paint a wall in 45 minutes, what fractional part of the wall can she paint in 9 minutes?
Since $9$ minutes is $1/5$ of $45$ minutes, we can find the fractional part of a wall that Heidi can paint in $9$ minutes by dividing the amount of wall that Heidi can paint in $45$ minutes by $5$. Since Heidi can paint a whole wall in $45$ minutes, it follows that she can paint $\boxed{\frac{1}{5}}$ of a wall in $9$ minutes.