Given:
\[
A = \begin{bmatrix} 3 & 2 \\ 1 & 5 \end{bmatrix}
\]
Step 1: Formula for Determinant
For a 2x2 matrix \( \begin{bmatrix} a & b \\ c & d \end{bmatrix} \), the determinant is:
\[
\det(A) = ad - bc
\]
Step 2: Substitute the Values
Identify \( a = 3 \), \( b = 2 \), \( c = 1 \), \( d = 5 \). Compute:
\[
\det(A) = (3 \cdot 5) - (2 \cdot 1) = 15 - 2 = 13
\]
Thus, the determinant is:
\[
\boxed{13}
\]