The probability of not opening the door for the first time is 1/2, and that of opening the door for the second time is 2/3.

1/2*2/3= 1/3

The probability of the second door opening is 1/3.

If you don't throw away the key you tried for the first time,

The probability of not opening the door for the first time is 1/2.

The probability of opening the door for the second time is 1/2

1/2* 1/2= 1/34

The probability of opening the door for the second time is 1/4.

I understand that after I can't open it for the first time, before I open the door for the second time, the four keys should be mixed together, instead of excluding the first key.

(2)

There are C5(3) = 10 ways for three people to be hired.

There are C4(2)=6 ways to get a position for girl K.

The probability of girl K getting the position is: 60%.

There are C5(3) = 10 ways for three people to be hired.

There are three ways for girls K and S to get jobs: C3( 1)=3.

The probability that girls K and S get jobs respectively is: 30%.

There are C5(3) = 10 ways for three people to be hired.

There are C4(2)=6 ways to get a position for girl K.

There are C4(2)=6 ways to get a position for girl S.

There are three ways for girls K and S to get jobs: C3( 1)=3.

The probability that a girl K or S gets the position is: (6+6-3)/ 10=90%.