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x in (-oo:+oo)
(1/4)*x+(1/3)*x = 840 // - 840
(1/4)*x+(1/3)*x-840 = 0
7/12*x-840 = 0 // + 840
7/12*x = 840 // : 7/12
x = 840/7/12
x = 1440
x = 1440
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