$$
\begin{array}{ll}
\text { Let } \overline{\mathrm{ab}}+\overline{\mathrm{cd}}=\overline{\mathrm{efg}} \\
\overline{\mathrm{ab}}=10, \overline{\mathrm{cd}} \text { can take } 90 \text { to } 99 & : 10 \text { values } \\
\overline{\mathrm{ab}}=11, \overline{\mathrm{cd}} \text { can take } 89 \text { to } 99 & : 11 \text { values } \\
\mathrm{M} \\
\overline{\mathrm{ab}}=99, \overline{\mathrm{cd}} \text { can take } 10 \text { to } 99 & : 90 \text { values } \\
(10+90) \times 81 \div+90 \times 9=4860 & \text { (values) }
\end{array}
$$
16. Fill in each square in $\square \square+\square \square=\square \square$ with a digit from $0, 1, 2, \ldots, 9$ (digits in the squares can be the same, and the leading digit of any number cannot be 0), so that the equation holds. There are $\qquad$ ways to fill in the numbers.