Many years ago, during the heydays of Trival Pursuit, Petro Canada, one of Canada's largest gas station chains, had a similar promotion whereby you would scratch quiz cards to win prizes.
However, for some strange reason, instead of getting participants to scratch what he/she thought was the correct answer, all of the answers were completely covered. Hence, you would have a question and a prize but you would have to GUESS where the right answer was hidden.
I'm not sure how they thought that this was in any way challenging unless we all had X-Ray Vision or a horseshoe up our butts. Suffice it to say, the promotion wasn't a hit with my dad who was a real whiz at Jeopardy! but lousy with luck.