# Substitution word problem

This question is very confusing to me. I understand can do normal substitution but word problems are more difficult.

The members of the city cultural center have decided to put on a play once a night for a week. Their auditorium holds 600 people. By selling​ tickets, the members would like to raise $3,900 every night to cover all expenses. Let d represent the number of adult tickets sold at ​$8.50. Let s represent the number of student tickets sold at

​$5.50 each. If all 600 seats are filled for a​ performance, how many of each type of ticket must have been sold for the members to raise exactly$3,900​?

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