First, convert the 20% to a real mathematical number. For percents, this is always done by dividing the 20% by 100%, or 20% / 100% = 0.200.
Second, find out what 20% of $600.25 is. This is the amount of the sale discount. This is always found by mulitplying 0.200 by the item's cost $600.25, like this:
0.200 x $600.25 = $120.05.
So for this sale, you'll save $120.05 on this item.
This means, the cost of the item to you is
$600.25 - $120.05 = $480.20.
Alternatively, you can think about it this way. The item is 20% off. This means you'll pay 80.000% of the total cost (100% - 20% = 80.000%).
Now what's 80.000% of the total cost?
0.800 x $600.25 = $480.20.
Just like the result above, the sale price on the item is $480.20.
Hope this helps you on your assignment! :)