a)
( a - b ) + ( b - c ) = 215 + 132
a - b + b - c = 347
a - c = 347
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b)
( a - c ) - ( b - c ) = 138 - 129
a - c - b + c = 9
a - b = 9
_____________________________
c)
( a - c ) - ( b + c ) = 18
a - c - b - c = 18
( a - b ) - 2 c = 18
72 - 2 c = 18
2 c = 72 - 18
2 c = 54
c = 54 : 2
c = 27