[quote name='dallascowboys']Why does everyone add percentages? If you want to get the net discount or added value, you have to multiply the percentages. 50% and an added 10% is an extra 55%, not 60%![/QUOTE]
Most places, including Gamestop.com, will multiply the percentages but Gamestop B&M does it differently.
With Gamestop Edge in-store, you always save an additional 10% off the regular used price, or get an additional 10% of the base trade-in value, regardless of what other promotions may be going on. The only exception is on "...free" offers, as you won't get an additional discount on top of the free item.
BTW, your math is wrong. If this promo were working the way you claim, Edge would give an extra 10% on 150% = extra 65% total.