To rewrite the equation \( 5y = 20x + 8 \) in terms of \( x \), Tilda first subtracts 8 from both sides:
\[ 5y - 8 = 20x \]
Next, to isolate \( x \), she needs to divide both sides of the equation by 20. This will give her \( x \) alone on one side:
\[ x = \frac{5y - 8}{20} \]
So, the number she should divide both sides of the equation by is 20.