To calculate the percentage of loss, we need to multiply the percentage decreases of each day.

First day: 90% decrease = 0.90 Second day: 80% decrease = 0.80

Total percentage of loss = (1 - (0.90 * 0.80)) * 100 = (1 - 0.72) * 100 = 0.28 * 100 = 28%

Therefore, the percentage of loss in this case is 28%.