Lets say I have the following data frame called `example`:

```
ID     amount_ID    
3      1
3      2
3      3
3      4
3      5
9      1
9      2
9      3
9      4
3      1
3      2
3      3

```

As you can see when a new `ID` starts the `amount_ID` starts counting from 1 again.

How can I add a column where I want a timespan of 100 for the `amount_ID` where the last `amount_ID` is 100. It should look like this:

```
ID     amount_ID   timespan
3      1           20         (because 1*100/5=20)
3      2           40         (because 2*100/5=40)
3      3           60         
3      4           80
3      5           100        (because 5*100/5=100)
9      1           25         (because 1*100/4=25)
9      2           50
9      3           75
9      4           100
3      1           33.33      (because 1*100/3=33.33)
3      2           66.67
3      3           100

```

I added some calculations so you can understand what I am trying to calculate.IMPORTANT: there are multiple rows with the same ID.

**Thanks!**