To calculate the overall percentage of loss, we need to find the combined effect of both stock falls.

Let’s assume the initial value of the stock is 100.

First, the stock falls by 90%, which means it is reduced to 10% of its original value. 10% of 100 = 10

Then, the stock falls again by 80%, which means it is reduced to 20% of its previous value. 20% of 10 = 2

Therefore, the final value of the stock after both falls is 2.

To calculate the percentage of loss, we need to find the difference between the initial value (100) and the final value (2), and then divide it by the initial value.

Loss = (100 - 2) / 100 = 98 / 100 = 0.98

The percentage of loss is 0.98 or 98%.