let

x = the number of students that study computer

y = the number of students that study economics

the number of students who study computer is twice the number who study economics:

x = 2y

the total number of students enrolled in these courses is greater than 600:

x + y >= 600

plug x = 2y into the inequality

2y + y > 600

by solving we find:

y > 200

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x > 2*200

x > 400

the number of students studying economics is greater than 600.