In a class of 40 students, 30 passed. What percentage passed?

Step 1: Identify the total number of students and the number of students who passed. Total number of students = $40$ Number of students who passed = $30$

Step 2: Calculate the fraction of students who passed. $\text{Fraction passed} = \frac{Number of students who passed}{Total number of students}$ $\text{Fraction passed} = \frac{30}{40}$

Step 3: Convert the fraction to a percentage. To convert a fraction to a percentage, multiply by $100\%$. $\text{Percentage passed} = \frac{30}{40} \times 100\%$ $\text{Percentage passed} = \frac{3}{4} \times 100\%$ $\text{Percentage passed} = 0.75 \times 100\%$ $\text{Percentage passed} = 75\%$

The percentage of students who passed is $\boxed{75\%}$.