Answer: 2.2 meters
Step-by-step explanation:
The data is:
The initial piece is 6.35 meters long.
We cut 25 pieces of 16.2cm each, so in pieces only we have:
25*16.2cm = 405cm
1m = 100cm
Then: 405cm = (405/100) m = 4.05m
And each time that we cut a piece, we have 4mm of waste.
Then we have 25 times the 4mm of waste, this means that the total waste is:
4mm*25 = 100mm
and 1000mm = 1m
100mm = (100/1000) m = 0.1m
So the total amount that was cut is:
4.05m + 0.1m = 4.15m
The length in the channel iron will be: 6.35m - 4.15m = 2.2 meters
, is not a mixed fraction. 
is a mixed fraction.