# Together 7440

There are a total of 58 pupils in the sixth and seventh grades and a total of 57 pupils in the sixth and eighth grades. There are a total of 59 pupils in the seventh and eighth grades. How many pupils are there in each class? How many students are there in all the classes together?

a =  28
b =  30
c =  29
s =  87

### Step-by-step explanation:

a+b=58
a+c = 57
b+c=59
s=a+b+c

a+b = 58
a+c = 57
b+c = 59
a+b+c-s = 0

Row 2 - Row 1 → Row 2
a+b = 58
-b+c = -1
b+c = 59
a+b+c-s = 0

Row 4 - Row 1 → Row 4
a+b = 58
-b+c = -1
b+c = 59
c-s = -58

Row 3 + Row 2 → Row 3
a+b = 58
-b+c = -1
2c = 58
c-s = -58

Row 4 - 1/2 · Row 3 → Row 4
a+b = 58
-b+c = -1
2c = 58
-s = -87

s = -87/-1 = 87
c = 58/2 = 29
b = -1-c/-1 = -1-29/-1 = 30
a = 58-b = 58-30 = 28

a = 28
b = 30
c = 29
s = 87

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