Student Enrollment Ratio and Future Calculation

In a certain high school, the number of students in grades 9, 10, and 11 are in the ratio 4:5:6, respectively. The total number of students in these grades is 240. Additionally, the school has a rising population with 20 more students enrolling in grade 9 for the next academic year. Assuming the ratio remains unchanged after this enrollment, how many students will there be in grade 10 next year?