Question

a weather balloon is filled with helium to a volume of 250 l at sea level, where pressure is 760 mm hg. assuming constant temperature, what will be the volume when the balloon raises to a height of 25 km, where the pressure is only 10 mm hg

Answer 1

You really need to find the caps key on your keyboard and use it. L is liters, not l, Hg is mercury, not hg, and sentences start with a cap letter.
Use P1V1 = P2V2
Post your work if you get stuck.