ANSWER
30 students
EXPLANATION
The cost per student varies inversely as the number of students.
Inverse proportion is written as:

Let the cost per student be y.
Let the number of students be x.
It will cost each student $250 if 24 students attend. This means that:

If the cost is down to $200, it means that y is now $200.
That is:

Therefore, 30 students could attend.
It would probably be x= 20 ×7
Take his age, subtract off 12 and you get the sister's age
(brother's age) - 12 = sister's age
x - 12 = 14
x - 12+12 = 14+12 ... add 12 to both sides to undo "minus 12"
x = 26
<h3>The brother is 26 years old</h3>