Let's break this down step by step:

1. Initially, the stock price is $100$.

2. The stock drops to 80%. That means the new price becomes:

   $100 \times 0.80 = 80$

3. Now, from this new price of $80$, the stock drops further to 90%. So the new price becomes:

   $80 \times 0.90 = 72$

4. The drop between 80% and 90% is the difference between the two prices:

   $80 - 72 = 8$

So, the stock dropped by 8 units between 80% and 90%.

Would you like more details or have any questions?

Here are some related questions to deepen your understanding:

1. What is the total percentage drop from the original stock price of 100 to 72?
2. If the stock rises back to 100% after reaching 72, what will the new price be?
3. What would be the stock price if it dropped to 70% of its initial value?
4. How does compounding affect the price when multiple percentage drops are applied sequentially?
5. What is the percentage drop from 80 to 72?

Tip: When dealing with sequential percentage changes, always calculate each step based on the new value rather than the original.