\[\boxed{\mathbf{1010\ (1010).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[1)\ \frac{12}{2400} = \frac{1}{200};\ \]
\[2)\ \frac{28 + 20}{2400} = \frac{48}{2400} = \frac{1}{50};\]
\[3)\ \frac{12 + 28 + 20}{2400} = \frac{60}{2400} = \frac{1}{40};\ \]
\[4)\ \frac{2400 - (12 + 28 + 20)}{2400} =\]
\[= \frac{2400 - 60}{2400} = \frac{2340}{2400} = \frac{39}{40}.\]