Answer:
0.0224 or 2.24%
Step-by-step explanation:
The probability that an intern uses drugs, given that the test is negative, is determined by the probability of false negative divided by the probability of a negative (true or false).
A false negative occurs when an intern who uses drugs (10%) takes the test and gets a negative. Since the test gets it right 80% of the time, the probability of this happening is:

A true negative occurs when an intern who does not use drugs (90%) takes the test and gets a negative. Since the test gets it wrong 3% of the time, the probability of this happening is:

Therefore, the probability that an intern uses drugs, given that the test is negative is:

The probability is 0.0224 or 2.24%.
I think this video will help you so much, it's about how to solve it from Khan Academy
https://www.khanacademy.org/math/pre-algebra/pre-algebra-exponents-radicals/pre-algebra-scientific-notation/v/scientific-notation-old
There would be 12 girls and 18 boys. Since the ratio is 2:3, there are 5 parts total (2+3=5) With 30 students, each part equals 6 students (30/5=6) Therefore the two parts that are girls would be 6 x 2, or 12 students, leaving the remainder of students, 18, to be boys