Because the sofa usually sells for $800, we know that's 100% of the price.
100% = $800
Let's find 1%.
1% = $800/100
1% = $8
Next, multiply $8 by 75 to find the percent of the sofas price when it's $600.
75% = $600
We now know that $600 is 75% of the sofas original price.
100% - 75% = 25%
The percent of decrease I'm the sofas sale price is 25%.
Hope I got that right, the question was worded a little strangley so I wasn't quite sure what you were asking :)