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For everyone to shake hands with everyone else, there are 21 handshakes.
(below copied from my answer of a similar question)
This is an arithmetic progression and can be solved with the equation Sn=(1+(n-1))(n-1)/2 where Sn is the total sum of handshakes for n people.
NB: I have used n-1 instead of n in the equation for the sum of an arithmetic progression, because you're not really going to shake hands with yourself, so you don't include the nth term, in this case 7.
Alternatively, you can solve this geometrically by drawing a heptagon, drawing lines between all the vertices, and then adding all the lines up.
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How many hand shakes would take place if 20 people shake hands once?
First person shakes hands 19 times, second person 18 etc, a total of 190.
If There are 5 people at a party Each person must shake hands once with every other person at the party How many handshakes does it take to do this?
10 times
How many times does the clock form right angle if the time is 325-600?
925
What is the fuedel pyramid?
a way of ranking people in feudal times (medieval europe)
What times on a clock form a straight line?
Starting with the hands in a straight line on the same side of the centre, consider how the hands move: At 12 o'clock they are in a straight line on top of each other. In 1 hour, the minute hand has moved a full circle, but the hour hand has moved forward a bit, so they are in a straight line again when the minute hand a moved a bit more. This will be repeated for each hour, but when the hour hand reaches the 12 again, the hands will have been in line 11 times in the 12 hours. So the hands are in line every 12/11 hours, or 1 hour 5 mins 27 3/11 seconds, giving the times as (rounded to nearest second): 12:00:00, 1:05:27, 2:10:55, 3:16:22, 4:21:49. 5:27:16, 6:32:44, 7:38:11, 8:43:38, 9:49:05, 10:54:33 When considering the hands in line on opposite side of the centre similar logic can be applied as before and so it is known that the hands will line up every 1 hour 5 mins 27 3/11 seconds, thus when the hands are lined up opposite the centre, the times will be (to the nearest second): 6:00:00, 7:05:27, 8:10:55, 9:16:22, 10:21:49, 11:27:16, 12:32:44, 1:38:11, 2:43:11, 3:49:05, 4:54:33
