To substitute \( c = 11 - p \) into the first equation \( 3c + 2.5p = 31 \), we start with the first equation:
\[ 3c + 2.5p = 31 \]
Now, substitute \( c \) with \( 11 - p \):
\[ 3(11 - p) + 2.5p = 31 \]
Now, distribute \( 3 \) in the equation:
\[ 33 - 3p + 2.5p = 31 \]
Combine like terms:
\[ 33 - 0.5p = 31 \]
Next, isolate \( p \) by first moving \( 33 \) to the right side:
\[ -0.5p = 31 - 33 \]
This simplifies to:
\[ -0.5p = -2 \]
Now, divide by \(-0.5\):
\[ p = \frac{-2}{-0.5} = 4 \]
We have found \( p = 4 \). If you need the value of \( c \), substitute \( p \) back into \( c = 11 - p \):
\[ c = 11 - 4 = 7 \]
So the solutions are \( p = 4 \) and \( c = 7 \).