You may have divided 10 by the 1 to simplify. However, 1 is not a factor, as it is being added to i. Treat the division as a fraction and rationalize the denominator:

$\displaystyle\frac{{10}}{{{1}+{i}}}=\frac{{10}}{{{1}+{i}}}\cdot\frac{{{1}-{i}}}{{{1}-{i}}}\usepackage{cmbright}\Huge\$

The correct answer is $\displaystyle{5}-{5}{i}\usepackage{cmbright}\Huge\$ .