To solve the given system of equations:
- Equation 1: \( x = 10 \)
- Equation 2: \( 3x + 5y = 20 \)
Since we already have the value of \( x \), we can substitute it into the second equation.
Substituting \( x = 10 \) into \( 3x + 5y = 20 \):
\[ 3(10) + 5y = 20 \]
This simplifies to:
\[ 30 + 5y = 20 \]
Now, subtract 30 from both sides:
\[ 5y = 20 - 30 \]
\[ 5y = -10 \]
Now, divide by 5:
\[ y = -2 \]
Thus, the solution \( (x, y) \) of the system of equations is:
\[ \boxed{(10, -2)} \]