#### Answer

The original price of the monitor was 135 dollars.

#### Work Step by Step

Let s represent the original price.
We can use the following guideline to solve this problem:
Original selling price - Discount = Discount sale price
s - 10% $\times$ s = 121.50
s - 0.1s = 121.50
0.9s = 121.50
Divide both sides by 0.9:
s = 135
The original price of the monitor was 135 dollars.