how many gummies are left if there is twenty in a bowl and someone eats eight and then someone replaces them with a thirty-six pack?

Pack? How many gummies in a pack, we need that info.

But if you mean gummies.

$${\mathtt{20}}{\mathtt{\,-\,}}{\mathtt{8}}{\mathtt{\,\small\textbf+\,}}{\mathtt{36}} = {\mathtt{48}}$$

48 Gummies.

48 gummies were left in the jar