A closed helium balloon is taking off from the Earth's surface, where the temperature and pressure are 300 K and 101 kPa respectively. Eventually, it will reach the point where the temperature and pressure are 258 K and 78 kPa respectively. Assuming that the balloon is of spherical shape initially with radius 10 m, find the factor by which its radius will increase. The balloon is in thermodynamic equilibrium with its environment. Do not take into account the surface tension of the balloon.