Answer:
0.4
Explanation:
consider this like some lines overlapping each other.
one line (A) is 0.5 long, the other (B) 0.4 long.
together they are 0.8 long.
that means that they overlap at a length of 0.1 (they share a segment 0.1 long).
of the combined line of 0.8 that has 0.1 of a joined segment, the complimentary part of B is therefore 0.4 (0.8 minus the original length of B).