[tex]\begin{gathered} \text{Let's she start with x.} \\ \text{She sold half of them, so remaing is}\Rightarrow\frac{x}{2} \\ \text{then she bought 11 more, now}\Rightarrow\frac{x}{2}+11 \\ \text{Then,}\Rightarrow\frac{x}{2}+11=41 \\ \frac{x}{2}=41-11 \\ \frac{x}{2}=30 \\ x=60\text{ stamp} \end{gathered}[/tex]
