Hi I need to find out a way to find average on data set when there multiple values but consider a specific value based on recent most

```
 Table 1
 ColumnA  ColumnB   ColumnC
   A        3         07/21/2017
   A        2         08/08/2017
   B        1          07/22/2017

```

Calculate averae of ColumnB

It should 2 from A, 1 from B, which should be (2+1)/2 =1.5 and not (3+2+1)/3=2. The criteria is based on recent most time stamp of each category of columnA