Starting with the equation:
\[ 5y = 20x + 8 \]
Tilda first subtracts 8 from both sides:
\[ 5y - 8 = 20x \]
To rewrite the equation in terms of \( x \), Tilda needs to isolate \( x \). Therefore, she should divide both sides by \( 20 \):
\[ x = \frac{5y - 8}{20} \]
Thus, the number she should divide both sides of the equation by in order to solve for \( x \) is 20.