$138.009 + 341.981 - 146.305 = 123.6 + ?$

For what value of $N$ will $270N$ be a perfect square,where $270N$ is a $4$-digit number?

What approximate value will come in place of the question mark $(?)$ in the given equation? (You are not expected to calculate the exact value)
$24.98^{2} \times \frac{16.02^{2}}{(7.98 \times 15.04)} \times 38.93 = 130 \times ?^{2}$

What mathematical operation should come at the place of $?$ in the equation $2 ? 6 - 12 \div 4 + 2 = 11$?

$\frac{69842 \times 69842 - 30158 \times 30158}{69842 - 30158} = ?$

$1+\frac{1}{2}+\frac{1}{4}+\frac{1}{7}+\frac{1}{14}+\frac{1}{28}$ is equal to