The final price of the sofa can be calculated by subtracting a decrease to the original price like this:
final price = original price - decrease
In this case, the original price was $520 but finally, the sale price was $416, then by replacing 520 for the original price and 416 for the final price we get:
416 = 520 - decrease
The decrease can be written as the multiplication of the original price by the percent of decrease that we will call "x", then we get:
416 = 520 - 520x
From this equation we can solve for x like this:
416 + 520x = 520 - 520x + 520x
416 + 520x = 520
416 - 416 + 520x = 520 - 416
520x = 104
520x/520 = 104/520
x = 104/520
x = 0.2 (20%)
Then the percent of decrease equals 20%
