\[\boxed{\mathbf{15}.}\]
\[1)\ 0,000321 = 3,21 \cdot 10^{- 4}\]
\[2)\ 0,000074 = 7,4 \cdot 10^{- 5}\]
\[3)\ 31\frac{2}{5} = 31,4 = 3,14 \cdot 10^{1}\]
\[4)\ 1401\frac{3}{25} = 1401\frac{12}{100} =\]
\[= 1401,12 = 1,40112 \cdot 10^{3}\]