Hi chengbin,

I set up two force diagrams for Allie and Beth.Cathy pulled with a force of70N so the opposing force has to be 70N.Theforces perpendicular to 70 must be equal and opposite By setting up two equations in xand y the lateral force is 18.5 .The 30N diagram gives opposing force 23.6 and the 50N diagram opposing force is 46.4 I got 59.8 for the angle between Allie and Beth's ropes using tangents to determine the acute angles in each force diagram

bjh