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.

Watch the video.