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25*(21/20-x) = 80*x // - 80*x
25*(21/20-x)-(80*x) = 0
25*(21/20-x)-80*x = 0
25*(21/20-x)-80*x = 0
105/4-105*x = 0
105/4-105*x = 0
105/4-105*x = 0 // - 105/4
-105*x = -105/4 // : -105
x = -105/4/(-105)
x = 1/4
x = 1/4
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