I have a data.table in R say df.

```
 row.number <- c(1:20)
 a <- c(rep("A", 10), rep("B", 10))
 b <- c(sample(c(0:100), 20, replace = TRUE))
 df <-data.table(row.number,a,b)
 df
   row.number a  b
1           1 A 14
2           2 A 59
3           3 A 39
4           4 A 22
5           5 A 75
6           6 A 89
7           7 A 11
8           8 A 88
9           9 A 22
10         10 A  6
11         11 B 37
12         12 B 42
13         13 B 39
14         14 B  8
15         15 B 74
16         16 B 67
17         17 B 18
18         18 B 12
19         19 B 56
20         20 B 21

```

I want to take the 'n' rows , (say 10) from the middle after arranging the records in increasing order of column b.