15. 50 students stand in a row facing the teacher. The teacher first asks everyone to report numbers from left to right as $1, 2, \cdots, 50$; then asks the students who report numbers that are multiples of 3 to turn around, and then asks the students who report numbers that are multiples of 7 to turn around. How many students are now facing the teacher?
【Analysis】There are $\left[\frac{50}{3}\right]=16$ students who are multiples of 3, and $\left[\frac{50}{7}\right]=7$ students who are multiples of 7. There are $\left[\frac{50}{3 \times 7}\right]=2$ students who are multiples of both 3 and 7. Therefore, there are $(16-2)+7-2=19$ students who turned around once, facing away from the teacher. The remaining 31 students either did not turn around or turned around twice, facing the teacher.