Nancy cut a piece of ribbon into three pieces. The first piece is twice as long as the third piece. The second piece is 15 cm shorter than the first piece. if the original ribbon is 300 cm long, how long is the third piece?
Let x be the length of the third piece.
The first piece is 2*x cm long.
The second piece is (2*x)-15 cm long.
The total length of the ribbon is x + 2*x + (2*x)-15 = 300.
Combining like terms, we get 5*x - 15 = 300.
Adding 15 to both sides, we get 5*x = 315.
Dividing both sides by 5, we get x = <<63=63>>63 cm. Answer: \boxed{63}.