The modulus ar absolute symbol...the two straight lines does not take into consideration the positive or negative signs and the answer always remains positive with the same numbers like in this case it is 21.
You can have flash cards. Put a number on them and give at least 10 people a number and to do that close your eyes and pick 10 people. Now shuffle your flash cards up. Oh, yeah, and you don't have you give them a number in person, like find a picture of smokers and non-smokers and tape a number on your computer on the spot of their shirt. So shuffle the cards and pick 5 of the numbers now you're done!