X / ( X - 3 ) - 5/4 = 0
( 4X - 5 * ( X - 3 ) ) / ( 4 * ( X - 3 )) = 0
( 4X - 5X + 15 )/ ( 4X - 12 ) = 0
( 15 - X ) / ( 4X - 12 ) = 0
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15 - X = 0
X = 15
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4X - 12 ≠ 0
X ≠ 3
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X - A = 0
A = X
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ОТВЕТ: А = 15