Let x be the length of the first part, then the length of the second part is 5x/4, and the length of the last part is 5x/4+0.8 metres. Since the total length of the three parts is 6.4m,
x+5x/4+5x/4+0.8=6.4,
14x/4=6.4-0.8=5.6,
14x=22.4, x=1.6m, the first part.
The second part is 5×1.6/4=2m long.
The third part is 2.8m long.
CHECK 1.6+2+2.8=6.4