This is one of the most important part of BCS preliminary Syllabus, specially for science students. 15 marks allotted for Mental ability. You need to answer six different topics-
1. Verbal Resoning
2. Problem Solving
3. Spelling And Language
4. Mechanical Resoning
5. Space Relation
6.Numerical Ability

I had experience of solving zigzag and Mogoj Dholai problems(two magazines published by two prominent BENgali Newspapers). Such habit can help you a lot.