PREP上的提, 这类提怎么算?

Working alone at is own constant rate, a machine seals K cartons in 8 hours, and a second machine seals K  in 4 hours. Together, what percent of catons sealed by machine working at the faster rate?

answer is 66又2/3%

 问题说的是，如果一起工作，较快的那个machine所加工的个数占K的多少percent？

{[K/(K/4+K/8)]*K/4}/K=2/3

多谢