Aschool has 10 classes with the same number of students in each class. one day, the weather was bad and many students were absent. 5 classes were half full, 3 classes were 3/4 full and 2 classes were 1/8 empty. a total of 70 students were absent. how many students are in this school when no students are absent?

Remark
The best way to do this is to figure out the fraction of empties and equate that number to 70.

Step One
givens 

x = number of students in each class.

Get fractions to represent the numbers who were absent.
Figure out the fractions for the empties.
5 classes. were 1/2 full. Therefore 5 classes were 1/2 empty. Each class has x students.

If 5 classes where 1/2 full, then these same classes were 1/2 empty. There are  
1 class = 1/2 x away.
5 classes = 5(1/2 x) away
5 classes = (5/2) x students away.

3 classes were 3/4 full. Therefore 3 classes are 1/4 empty
3(1/4)x = (3/4)x total number of students absent in 3 classes.

2 classes were 1/8 empty. 
2*(1/8)*x = (1/4) x

Step Two
Add your absences. Equate to 70
(1/4)x + (3/4)x + (5/2)x  = 70 

Step Three
Find your common denominator
the common denominator is 4. Convert all fractions to something over 4.
1/4  has a denominator of 4
3/4 has a denominator of 4
5/2 = 5*2/2*2 = 10/4

So the equation becomes.
(1/4)x + (3/4)x + (10/4)x = 70

Step Four
Solve for x
(1 + 3 + 10)x/4 = 70      Collect like terms on the left
(14x)/4 = 70                  Multiply by sides by 4
14x = 70 * 4
14x = 280                     Divide by 14
x = 280/14
x = 20

Answer
Each class contains 20 student.
10 classes contain 20 * 10 = 200 students.