Mind Puzzle : What is black when you buy it

Let's see how intelligent you are!
Riddle Question

What is black when you buy it,
red when you use it,
and gray when you throw it away?

Let's see how intelligent you are!
Solve this math problem to see answer.
Riddle Answer

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It belongs to Logical category and tagged with logic questions logic riddles brain puzzles tricky questions logic puzzles what am I free iq test logic questions with answers mind puzzles