According to Charles' law, what is the possible minimum temperature?

$A$ balloon contains $500 \ m^3$ of helium at $27^{\circ}C$ and $1 \ atm$ pressure. The volume of the helium at $-3^{\circ}C$ temperature and $0.5 \ atm$ pressure will be ..... $m^3$.

The number of molecules in $1 \, cm^{3}$ of an ideal gas at $N.T.P.$ is:

At constant temperature,increasing the pressure of a gas by $5\%$ will decrease its volume by ..... $\%$

An air bubble of volume $2.9 \ cm^3$ rises from the bottom of a swimming pool $5 \ m$ deep. At the bottom of the pool, the water temperature is $17^{\circ} C$. The volume of the bubble when it reaches the surface, where the water temperature is $27^{\circ} C$, is . . . . . . $cm^3$. ($g = 10 \ m/s^2$, density of water $\rho = 10^3 \ kg/m^3$, and $1 \ atm = 10^5 \ Pa$)

Which quantity is constant for two gases of equal volume at the same pressure and temperature?