10 friends are shaking hands because they have not seen each other for long time. Every single friend shakes hand from other. How many times does one friend shake hands? How many shakes of hands are there (from all of the friends)?
Every friend shakes 9 hands and the total number of shakes is 45.
The first friend will shake 9 hands.
The second friend will shake the hand with the first, and then he will shake other 8 hands.
The third friend has already shaken hands with the first two friends, so he will shake other 7 hands.
And so on.
At the end, every friend shakes 9 hands, but the total number of shakes are (9+8+7+6+5+4+3+2+1)=45.