这两句英语中reduce为什么接介词? reduce by 和 reduce to有什么区别?

The price was reduced by 30 percent.价格降低了30%。
The loss of metal has been reduced to less than 20 percent.金属的损失率已降到20%以下。
reduce有及物用法，为什么他们都须解接介词呢?
接介词的意思不一样。没有介词，后面就没有宾语了。
The price was reduced.价格降低了。
The loss of metal has been reduced.金属的损耗减少了。
reduce... by...减少了……（表示减了多少）（by表示程度）
reduce... to...减少到……（表示减到多少）（to表示结果）
例：
No-tillage without residue cover can reduce the erosion by 21.6%. 
另外，仅有免耕而没有秸秆覆盖能减少21.6％的沙尘量。
When that starts to happen we will reduce the price to 1.15 . 
一有种迹象发生，我们就会把价格降到1.15英镑。

意思不一样
reduce作及物动词时是降低的意思
如：
This operation makes sure it will reduce the risk of being cheated.这种方案确保了它将会降低被欺诈的风险。
reduce to 降低到…
如：
The index has reduceed to 80% of it used to be .
指数降为原来的百分之八十了。
reduce by 降低了（多少）
The number of classmates in this semester reduce by
5 people .
这学期学生人数减少了5人。

前者“以百分之三十”，后者“到百分之二十”