Tickets to a show cost $10 in advance and $15 at the door. if 120 tickets are sold for a total of $1,390, how many of the tickets were bought in advance?

Let x = number of tickets bought in advance
      y = number of tickets bought at the door

We have two unknowns therefore we need to equations to solve the problem. We do as follows:

x + y = 120
10x + 15y = 1390

SOlving simultaneously,
x = 82
y = 38

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