You need to make a series of equations from what you are given first. I am going to use the first letter of each of the names to represent the length of that persons wire.
1/2s=2/5d
3c=s
s+d+c=6 ft
Okay. Now you can combine the first two equations knowing what s equals:
1/2(3c)=2/5d
d=15c/4
Now you have d=15c/4 and s=3c, so you can replace d and s in the third equation.
3c+15c/4+c=6
Then solve for c and plug it into the equation 3c=s to find the length of sarah's wire.