12) 3 2/5 · 1 1/84 =

\(3\frac{2}{5}=\frac{17}{5}\), \(1\frac{1}{84}=\frac{85}{84}\).

\(\frac{17}{5}\cdot\frac{85}{84}=\frac{289}{84}=3\frac{37}{84}\).

Ответ: 3 37/84.