3. (2,61 − 4x) : 0,03 + 4,6 = 90

\[\frac{2.61-4x}{0.03}+4.6=90\]

\[\frac{2.61-4x}{0.03}=85.4\]

\[2.61-4x=85.4\cdot0.03=2.562\]

\[4x=2.61-2.562=0.048,\quad x=0.012\]

Ответ: \(x=0.012\).