1st Day at school

1. Jan 10, 2007

powergirl

It was Cheryl's first day at school. The teacher suggested that it would be a good idea for each child to meet every other child in the class. The teacher said, "When you meet, please shake hands and introduce yourself by name."

If there were 15 children in the class, how many total handshakes were there?
:uhh:

2. Jan 10, 2007

dontdisturbmycircles

$$\sum_{i=1}^{15}i$$

3. Jan 10, 2007

powergirl

No.....Slightly wrong

4. Jan 10, 2007

Hootenanny

Staff Emeritus
14.............

5. Jan 10, 2007

powergirl

14...................?

6. Jan 10, 2007

dontdisturbmycircles

It's not 120?

7. Jan 10, 2007

Hootenanny

Staff Emeritus
I thought it was only Cheryl's first day, therefore all the other children would already know each other.

If not the answer must be 210.

8. Jan 10, 2007

powergirl

no.......sorry

9. Jan 10, 2007

dontdisturbmycircles

Lol I thought way too fast... 105!

10. Jan 10, 2007

powergirl

Ya..Now it's Correct........

11. Jan 10, 2007

Hootenanny

Staff Emeritus
hmm, could someone explain this one to me?

Hey dontdistrbmycircles :)

12. Jan 10, 2007

powergirl

may I do that?

13. Jan 10, 2007

dontdisturbmycircles

Sure, the first boy shakes 14 hands right? Then the next shakes 13 until you get to 1, my original sum should have read $$\sum_{i=1}^{14}i$$ :P

Keeps going down one because the same person does not shake the same person's hand twice.

14. Jan 10, 2007

powergirl

Ya right....
The class has 15 children. The first

child shakes hands with the other 14

children. The second child has already

shaken hands with the first child, and so

has to shake hands with only the other 13

children. In this manner, the

second-last child has to shake hands with

only one child, and the last child has

already met all the children. Thus, the

number of handshakes is

14 + 13 + ........ + 2 + 1 = 105.

If there were 15 children in the class,

then there were 105 total handshakes.

15. Jan 10, 2007

Hootenanny

Staff Emeritus
Damn it :grumpy:. Thanks!

16. Jan 10, 2007

powergirl

Oh....Am I correct?