To find out how many students 28 represents 70% of, you can set up the equation:
\[ 0.70 \times x = 28 \]
where \( x \) is the total number of students. To solve for \( x \), divide both sides by 0.70:
\[ x = \frac{28}{0.70} \]
Calculating that gives:
\[ x = 40 \]
So, 28 students is 70% of 40 students.