Total length of the ribbon = 6 m = 600 cm
length of first piece = 1 m 50 cm = 150 cm
length of the second piece = 2 m 40 cm = 240 cm
Total length of the ribbon
= length of first piece + length of the second piece + length of the third piece
= 150 + 240 + length of the third piece
= 390 cm + length of the third piece
600 cm = 390 cm + length of the third piece
length of third piece = 600 - 390 = 210 cm
= 200 cm + 10 cm
= 2 m 10 cm