Since we know that Ms. Kwan made 4 more sandwiches in case there are guests, she made [tex]49-4=45[/tex] sandwiches for he students.
We also know that she made [tex]1 \frac{1}{2} [/tex] for each student, so to find we many sandwiches she made for her students, we are going to divide 45 by [tex]1 \frac{1}{2} [/tex]:
[tex] \frac{45}{1 \frac{1}{2} } = \frac{45}{1.5} =30[/tex]
We can conclude that since she made 30 sandwiches for her students, there are no more than 30 students in her class, or in mathematical notation: [tex]s \leq 30[/tex].