A tile depot's board reads: 'Buy 5 tiles, get 2 tiles free.' A customer who takes full advantage of the offer gets an effective discount of what percent?

He pays for 5 tiles and carries away 7, so 2 tiles out of the 7 he receives are free: 2/7 = 28.57%. (Comparing the 2 free tiles with the 5 paid for gives 40%, but a discount percent is always measured on what the 7 tiles would normally have cost. 33.33% would be right for 'buy 4, get 2 free' and 25% for the familiar 'buy 3, get 1 free'.)