definition
In the first stage, the number of a quantity is Y, and in the second stage, Y is divided into two parts, Y=Y1+Y2, where Y1 is the part of the quantity that still exists in the second stage, and Y2 is the part that has disappeared. Then the retention rate of this quantity in these two stages is u=Y1/Y.
For example, a company had 100 employees at the end of 2008. A total of 23 of these employees no longer worked in the company in 2009. Ask the company what the retention rate of the old employees in 2009 is.
Known: Y=100, Y2=23,
Then: Y1=Y - Y2=100 - 23=77
Retention u=77/100=77%
