Jonny's first step in solving the equation \( 8 + 3n = 20 \) is to subtract 8 from both sides in order to isolate the term with \( n \). The goal is to simplify the equation and make it easier to solve for \( n \). By subtracting 8 from both sides, he effectively removes the constant term on the left side:
\[ 8 + 3n - 8 = 20 - 8 \]
This simplifies to:
\[ 3n = 12 \]
Now the equation is simpler and can be solved for \( n \) by dividing both sides by 3. Subtracting 8 is a key step in rearranging the equation to focus on the variable \( n \).