It takes 3 1/3 kilometers of thread to make 3 1/2 boxes of shirts. How many kilometers of thread will it take to make 6 boxes?

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