The circumference is 3 inches and the length of tube is 16 inches. The string is wrapped around symmetrically around the tube. What is the length of the string?
If the string covers the entire surface area, consider it a bunch of 3-inch circles.
Now, you have to know the diameter of the string. If it is 1/20 inch, then there will be 16*20 = 320 loops.
So, nail that down, and you can answer the question.