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x in (-oo:+oo)
x/21 = 25/15 // - 25/15
x/21-(25/15) = 0
x/21-5/3 = 0
1/21*x-5/3 = 0 // + 5/3
1/21*x = 5/3 // : 1/21
x = 5/3/1/21
x = 35
x = 35
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