Tickets to a concert cost $10 in advance and $15 at the door, so if 120 tickets were sold for a total of $1390, how many of the tickets were purchased in advance?
Let X be the number of tickets sold in advance
Let Y be the number of tickets sold at the door
Then,
X + Y = 120 (total number of tickets sold)
X*$10 + Y*$15 = $1390 (total revenue from sales)
Substituting for X = 120 - Y into the money equation,
(120 - Y)*10 + Y *15 = 1390
1200 - 10Y + 15Y = 1390
5Y = 190
Y = 38
X = 120 - 38
X = 82
Answer: 82 tickets sold in advance, 38 tickets sold at the door.