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(1/5)*(100-x) = 12 // - 12
(1/5)*(100-x)-12 = 0
1/5*(100-x)-12 = 0
1/5*(100-x)-12 = 0
8-1/5*x = 0
8-1/5*x = 0
8-1/5*x = 0 // - 8
-1/5*x = -8 // : -1/5
x = -8/(-1/5)
x = 40
x = 40
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