Objective 5:

1.  This law shows the relationship between pressure and volume of a gas.
http://en.wikipedia.org/wiki/Boyle%27s_Law
2.  As the balloon rises it begins to get bigger which means that they can't fill a balloon completely up or else it will pop. 
http://wiki.answers.com/Q/Why_do_most_helium_filled_balloons_burst_as_they_ascend_to_high_altitudes
3.  PV = k is the formula.  P=pressure V=volume of gas k=constant value of pressure and volume
http://wiki.answers.com/Q/What_is_the_formula_of_Boyle's_law&alreadyAsked=1&rtitle=What_is_the_formula_for_Boyle%27s_Law
4.  They use it when they do oxegen tanks because when they find the pressure in the tank they can see how much is actually left in the tank.
5.  Scuba divers use it for their oxegen tanks because just like doctors they can see how much oxegen they have left.