Answer:
5 5/7 km
Step-by-step explanation:
We assume the length of thread is proportional to the number of boxes of shirts. If L is the thread length required, then that assumption means
... L/(6 boxes) = (3 1/3 km)/(3 1/2 boxes)
Multiplying the equation by (6 boxes), we get
... L = (3 1/3 km)(6/(3 1/2)) = (10/3 km)(6/(7/2))
... = (10/3 km)(12/7) = 40/7 km = 5 5/7 km
