When it comes to the translation of the decrease of numbers, it should not be translated into “减少了多少多少倍”. It should use the fractional numbers to translate it, that is “减少了几分之几，或减少到几分之几”. It is because that in Chinese we usually prefer the latter, which is more native.

The first expression is:

“…… + 减少意义的谓语 + by a factor of n或by n times”。

“成 n 倍地减少”，即减少前的数量为减少后的倍数。

(1) The automatic assembly line can shorten the assembling period (by) ten times.

自动装配线能够缩短装配期十分之九。

(2) This metal is three times as light as that one.

这种金属比那种金属轻三分之二 。

The second expression is:

“n times + 减少意义的比较级”，其意义和 n times as light as 等价。注意不要被字面迷惑。

This metal is three times lighter than that one.

这种金属比那种金属轻三分之二 。

Note:

decrease（by）3 times

应是“减少2/3”，而不是“减少3/4”。