SQL: Using index in select for performance - question?

I am using informix online 7.1 Have a table with 3 columns 30,000 rows. Have index on trip_level and date (see below) if I do a select ... where trip_level = 1 and time >= "1996-08-21 09:34:07"; or select ... where time >= "1996-08-21 09:34:07" and trip_level = 1; it takes 17 seconds. if I do a select ... where time >= "1996-08-21 09:34:07"; it takes 1 second. It looks like the server only uses the time index in the last case. How can I get it to use the time index for all cases. Table info: Column name Type Nulls serial_number integer no trip_level smallint no time datetime year to second no Index name Owner Type Cluster Column 141_224 informix unique No serial_number ix148_5 informix dupls No trip_level ix148_6 informix dupls No time Thnaks.