The trifecta at most racetracks consists of selecting the first-, second-, and third-place finishers in a particular race in their proper order. if there are eleven entries in the trifecta race, how many tickets must you purchase to guarantee a win?

Since you have 11 entries, you have 11 possible winners.

For each winner, you have 10 possible second-place finishers.

For each pair first-second place, you have 9 possible 3rd place finishers.

The possibilities are 11*10*9 = 990

Therefore, you have to buy 990 tickets.