First, the length of the overlapping fastening part of the left iron ring and the middle iron ring is 0.5cm+0.5cm= 1cm.

The overlapping length of the right iron ring and the middle iron ring is also 0.5cm+0.5cm= 1cm.

If you want to know the total length of three iron rings, you only need to know the length of the non-overlapping part of the middle iron ring, that is,

4cm- 1cm- 1cm=2cm

Therefore, the total fastening length of the three iron rings =4cm+2cm+4cm= 10cm.