The first thing we have to do is identify the equations:
[tex]\begin{gathered} M\to Mother\text{ age} \\ D\to\text{ Daughter age} \end{gathered}[/tex][tex]\begin{gathered} M=27\to(1) \\ (M_{}-1)=2D\to(2) \end{gathered}[/tex]We replace the equation 1 by 2 to find the age of the daughter
[tex]\begin{gathered} (27-1)=2D \\ 26=2D \\ D=\frac{26}{2} \\ D=13 \end{gathered}[/tex]