Algebra-1

A school is organizing a field trip to two museums, X and Y. The ticket for Museum X is 15 dollars per student, and for Museum Y is 20 dollars per student. The school can take a maximum of 40 students, and the number of students visiting Museum X must be at least twice the number visiting Museum Y. Let x be the number of students visiting Museum X and y be the number visiting Museum Y (x, y are non-negative integers). __________.