like they established already that 6000 comes from the 1200 students with the total of 5 classes among them....

soo,,,,,, and we originally know that in EACH class there is a total of 30 students so...... in order to find the ACTUAL number of classes

you had to take that 6000 and divide it by 30 bc you know that you have 6000 students and you also know that each class has thirty so...

30x = 6,000

think of it like that :D