Senior three students line up at intervals, that is to say, between two senior three students, one seat is reserved for senior one or senior two students.

Number of permutations: a (3,3)

Senior one students have three positions to choose from: A(3, 1).

The remaining two positions are for senior two, A(2, 2).

The first senior three student has two positions to choose from, the first and the second.

So the arrangement method: A (2,2) * A (3,3) * A (3,1) * A (2,2) = 72 kinds.

2、

There are five cards in five bags of food. If you have 3 different cards, you can win the prize.

Winning probability: 3/5

3. the probability is the probability that this line is spades or hearts K.

Spades have 13, and hearts have only 1, making a total of 14.

Probability: 14/52=7/26.