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题库 / GMATLA-PS-87
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
~$1\frac{1}{4}$~
~$1\frac{1}{2}$~
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