Bunuel wrote:
Tough and Tricky questions: Combinations.
If 11 persons meet at a reunion and each person shakes hands exactly once with each of the others, what is the total number of handshakes?
A) 11 × 10 × 9 × 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1
B) 10 × 9 × 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1
C) 11 × 10
D) 55
E) 45
Kudos for a correct solution.Source: Chili Hot GMAT
its the sum from 1 to 10, coz the person can't shake hands with himself then each next to him loses 1 chance to shake hands because he's shaken hands with the one counted:
10+9+8+......+1=(10+1)*10/2=55
this shortcut applies to other questions the like, such as games between two teams blablabla