http://www.mathsisfun.com/algebra/sigma-notation.html This link explains clearly what is that symbol. FYI

Mysql equivalent:

    Select count(*)
    from student
    where number<3000;

Font size is too little in the expression

σ(Number<3000)(Student) This Unary Relational Algebraic operation means, select number of students below 3000. So, the answer is 2.

In this question the question just simple and based on the condition that σ(Number<3000)(Student) ,according to this, in the Number Column count the value which is less than 3000.