To rewrite the equation \( 5y = 20x + 8 \) in terms of \( x \), Tilda can start by subtracting 8 from both sides.
Starting with: \[ 5y = 20x + 8 \]
Subtracting 8 from both sides gives: \[ 5y - 8 = 20x \]
Now, to isolate \( x \), Tilda should divide both sides by 20.
Thus, the answer is: 20.