Answer : 3. "78%"
Explanation :

To solve this problem, let's first represent the number of boys and girls in terms of a common variable. Let's say there are 3x boys and 2x girls.

Given that 20% of the boys and 25% of the girls are adults, we can calculate the number of adults among them.

Number of adult boys = 20% of 3x = (20/100) * 3x = 0.2 * 3x = 0.6x Number of adult girls = 25% of 2x = (25/100) * 2x = 0.25 * 2x = 0.5x

So, the total number of adult students = 0.6x (boys) + 0.5x (girls) = 1.1x

Now, the total number of students in the college = 3x (boys) + 2x (girls) = 5x

The percentage of students who are not adults = (Total number of non-adult students / Total number of students) * 100%

Since the total number of adult students is 1.1x, the total number of non-adult students = Total number of students - Total number of adult students = 5x - 1.1x = 3.9x

So, the percentage of students who are not adults = (3.9x / 5x) * 100% = (3.9/5) * 100% = 78%

Therefore, the percentage of students who are not adults is 78%.