You have two boxes of colored pens. The first box contains a red pen, a blue pen, and green pen. The second box contains a yellow pen, a red pen, and a black pen. What is the set that represents all the pens?

Answer 1

that would be the set of all the pens:
{red,blue,green,yellow,red,black}

There are 2 red pens in the set of all the pens, but if you just want a set that contains all the unique colors, then red will occur only once.