Notice that the price of the tickets are:
for teachers: $12, and for students $10 each.
The group has 16 students, so the total for the students' tickets is:
$10 * 16 = $160
There are three teachers total (Ms Debbie and two more) the the amount spent in teachers' tickets is:
$12 * 3 = $36
The total spent if tickes is the addition of the two values we just calculated : $160 + $36 = $196
Then, after she pays for the tickets, the amount left is:
$240 - $ 196 = $44