Aparticular english class has 40 students. as the students leave the room, only 28 students are left after 3 seconds. what is the rate of change of the number of students in the classroom each second? -4 students per second 4 students per second 12 students per second -12 students per second

Find how many students left:
40-28 = 12 left 

divide number of students that left by the amount of time :

12 / 3 = 4 students left per second

because the class room got smaller ( students left, the rate in the class room is declining so the answer would be a negative number)
 answer is -4 students per second