The most important step is to identify the unknown quantity yiou are trying to find. In this case, it's the number of tickets they need to sell. So let x = the number of tickets they need to sell. The amount of money they get is equal to $15 times x ($15x).
$15x >= $1000
Solve for x. If you get a fraction or decimal, round up to the next whole number - you can't sell part of a ticket.