1. **Problem statement:** A population of 600 students is divided into two groups: 330 from School A and 270 from School B. A sample of 100 students is needed using proportional stratified sampling. We need to find how many students should be selected from each school. 2. **Formula and explanation:** In proportional stratified sampling, the sample size from each group is proportional to the size of that group in the population. The formula for each stratum is: $$ n_i = \frac{N_i}{N} \times n $$ where: - $n_i$ is the sample size from group $i$, - $N_i$ is the population size of group $i$, - $N$ is the total population size, - $n$ is the total sample size. 3. **Calculate sample size for School A:** $$ n_A = \frac{330}{600} \times 100 = 0.55 \times 100 = 55 $$ 4. **Calculate sample size for School B:** $$ n_B = \frac{270}{600} \times 100 = 0.45 \times 100 = 45 $$ 5. **Answer:** - Number of students selected from School A: 55 - Number of students selected from School B: 45 This ensures the sample reflects the population proportions accurately.