A balloon filled with helium occupies 21.0ft³ at 55.0°F. Find the temperature of the helium if the volume of the balloon decreases to 11.0ft³. (Assume that the pressure is held constant.)

Assuming pressure is held constant,the question reduces to a ratio and proportion type of question where;

At constant pressure,

21.0ft³-55.0°F

11.0fy³=11.0ft³/21.0ft³×55°F

The temperature is 28.809°F≈29°F

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)