If 100 students make up 40% of the 6th grade population, then 50 students make up 20% of the 6th grade population. 20% × 5 makes 100% so if your multiply 50 by 5 you'd get your population. 50 × 5 = 250 sixth-graders
If one hundred sixth-grade students represent 40% of the total, you need to find the 100% so, multiply 100% by 100 students and then divide by 40% and you'll have the total sixth-grade students.