Use three dice.

Have a friend roll the dice and then stack them one on top of the other.

Tell your friend that you can not see five faces of the dice.

The 5 numbers  you cannot see are :
the bottom of the top dice
the top of the second dice
the bottom of the second dice
the top of the last dice
the bottom of the last dice

You will now predict the total of the five hidden numbers.

In your head subtract the very top face of the three dice from 21.
The answer will be the total of the five hidden numbers.
 
In the video, the top number is 4, therefore other five will add up to (21 - 4 = 17)  17

Ps:
You can also do this math trick with two dice. Simply subtract the top number from 14.