Well, let's look at it if everyone was in a line. The first executive would step out of the line and shake 4 hands, and be done. Then, the second executive would shake 3 hands (since the first executive has already shaken hands with them, and there are only 4 left including them). The third would shake 2 new hands and the fourth would shake the fifth's hand, getting (4+3+2+1)=10. Since it's before and after, we multiply by 2 to get 20