$9999 + 8888 + 777 + ? = 19700$

Estimate the value of $\sqrt{624} \times \sqrt{63} + \sqrt{398} \div \sqrt{17} = ?$

If $50$ is subtracted from two-third of a number,the result is equal to the sum of $40$ and one-fourth of that number. What is the number?

$29.8 \%$ of $260 + 60.01 \%$ of $510 - 103.57 = ?$

The least positive integer that should be subtracted from $3011 \times 3012$ so that the difference is a perfect square,is

$9067 + 2065 - 8400 + 3045 - 1520 = ?$