Starting with the equation:
\[ 5y = 20x + 8 \]
After subtracting 8 from both sides, we have:
\[ 5y - 8 = 20x \]
Now, to isolate \( x \), we need to divide both sides by 20, since we want to express the equation in terms of \( x \). Thus, the number Tilda should divide both sides by is:
\[ 20 \]