If $\frac{a}{a+b}=\frac{17}{23},$ what is $\frac{a+b}{a-b}$ equal to?

What should come in place of the question mark $(?)$ in the following equation?
$\sqrt{575} \div ? \times 14.98^{2} = 450$

Simplify: $2 \div [2 + 2 \div \{2 + 2 \div (2 + 2 \div 3)\}]$

$A$ person was to multiply a fraction by $\frac{6}{7}$. Instead,he divided the fraction by $\frac{6}{7}$ and got an answer which exceeds the correct answer by $\frac{1}{7}$. The correct answer is:

If $M = \left(\frac{3}{7}\right) \div \left(\frac{6}{5}\right) \times \left(\frac{2}{3}\right) + \left(\frac{1}{5}\right) \times \left(\frac{3}{2}\right)$ and $N = \left(\frac{2}{5}\right) \times \left(\frac{5}{6}\right) \div \left(\frac{1}{3}\right) + \left(\frac{3}{5}\right) \times \left(\frac{2}{3}\right) \div \left(\frac{3}{5}\right)$,then what is the value of $\frac{M}{N}$?

$12.1212 + 17.0005 - 9.1102 = ?$