Try this:
SELECT *
FROM (
SELECT CONVERT(INT, LEFT(PARTICULARS, CHARINDEX('of',PARTICULARS)-1)) AS ID, PARTICULARS
FROM UNPAID_FORM
) AS T
ORDER BY ID
Result:
1 1 of 20% item
2 2 of 20% item
5 5 of 20% item
10 10 of 20% item
[EDIT #1]
As mentioned, it's Firebird database engine.
SELECT * FROM (
SELECT CAST(LEFT(PARTICULARS, POSITION(' of' IN PARTICULARS)) AS INT) AS ID, PARTICULARS
FROM UNPAID_FORM
) AS T
ORDER BY ID
Many interesting functions for Firebird, you'll find here:
http://www.volny.cz/iprenosil/interbase/ip_ib_code_string.htm[
^]