減少了5/6

There is a n-fold decrease/reduction… 
應譯為：減至1/n [或：減少（n一1）/n]。（17） 
這個句型還有其他一些形式， 

Eg. A rapid decrease by a factor of 7 was observed． 
發現迅速減少到1/7。 

Eg. The principal advantage of the products is a two-fold reduction in 
weight. 
這些產品的主要優點是重量減輕了1/2。