Please advice on a query

I have a table more or less like this:

-----------------------------------------------------
crs_id                some data                req
-----------------------------------------------------
25                       aaaaaaaa                 21
25                       aaaaaaaa                 19
17                       ccccccccc                 null
8                         dddddddd                 null
19                       eeeeeeee                 15
19                       xxxxxxxxx                  37
15                       fffffffffffffffff                  10
10                       gggggggg                   null

I need to delete rows according to the following criteria:

* Row with "crs_id = 25" must be deleted (only the second row) because "req
= 19" and there is a row with "crs_id = 19"

* Row with "crs_id = 19" must be deleted (only the first row) because "req
= 15" and there is a row with "crs_id = 15"

* Row with "crs_id = 15" must be deleted because "req = 10" and there is a
row with "crs_id = 10"

I have been struggling with this issue without a successful result.

Respectfully,
Jorge Maldonado