# Homework Help: Probability expected value

1. Apr 13, 2013

### ParisSpart

16 people Meet together at a party. Each pair of these individuals decide to chance whether you hailed by handshake or not (probability handshake = 0.40). On average, how many handshakes occur?

X=handshakes if 1 and no handshakes if 0
X=X1+X2+X3+X4+X5+X6+X7+X8 and E(X)=E(X1)+....+E(X8)=8*0.40 but its says tha its not correct where is wrong?

2. Apr 13, 2013

### Ray Vickson

You say that 16 people can be paired in only 8 ways?

3. Apr 13, 2013

### ParisSpart

256?

4. Apr 13, 2013

### Ray Vickson

Work it out. Don't guess!

5. Apr 13, 2013

### ParisSpart

each person can be paired with other 15 people so we have 15*16=240 but 240*0.40 its not correct where is my wrong?

6. Apr 13, 2013

### ParisSpart

the way thst i solve this is wrong?

7. Apr 13, 2013

### Ray Vickson

8. Apr 13, 2013

### ParisSpart

this means that you cannot help?

9. Apr 13, 2013

### haruspex

If you choose 2 people from 16, you can choose the first in 16 ways and the second in 15 ways. But for a given pair, you don't care what order you chose them in. Try it with smaller numbers, like 2 from 2, 2 from 3...

10. Apr 14, 2013

### HallsofIvy

If you honestly have no idea how to even approach this problem, where did you get it? If this really is homework, clearly your teacher expects you to know things that you apparently don't. You really need to talk to your teacher about this.