The greatest number among $3^{50}, 4^{40}, 5^{30}$ and $6^{20}$ is

If doubling a number and adding $20$ to the result gives the same answer as multiplying the number by $8$ and taking away $4$ from the product,the number is

$\left[ {10 + \left\{ {4 \times \left( {\overline {\frac{2}{3} + \frac{1}{4}} \times \sqrt {\frac{{144}}{{121}}} + 23} \right) \div 12 + 5} \right\} - 3} \right] = ?$

$(2 \sqrt{392} - 21) + (\sqrt{8} - 7)^{2} = (?)^{2}$

$\frac{753 \times 753 + 247 \times 247 - 753 \times 247}{753 \times 753 \times 753 + 247 \times 247 \times 247} = ?$

$7372 \times 7372 + 7372 \times 628 = ?$