Working alone at their respective constant rates, machine A and machine B can fill a certain order in 3 hours and 6 hours, respectively.  If the two machines work simultaneously at their respective constant rates, how many hours does it take the two machines to fill ~$\frac{1}{2}$~ of that order?

~$\frac{1}{2}$~

~$\frac{3}{4}$~

1

~$1\frac{1}{4}$~

~$1\frac{1}{2}$~