Note: It seems you mean a piece of string is 3.5 long. So, I am assuming that a piece of string is 3.5 cm long.
Answer:
140 cm of the original piece is left after Joe cuts three pieces.
Explanation:
Given the piece of string
As we know that
so the length of a piece of string = 350 cm
As joe cuts three pieces from it, and each 70 cm long.
so the total length of three pieces which were
cut by joe = 70cm + 70cm + 70cm = 210 cm
So, subtracting the total length of the three pieces that were cut from the total length of the string:
350 cm - 210 cm = 140 cm
Therefore, 140 cm of the original piece is left after Joe cuts three pieces from it.