3.78 The two couples shown are to be replaced with a single equivalent cou- ple. Determine (a) the couple vector representing the equivalent couple, (b) the two forces acting at B and C that can be used to form that couple. 3.79 Solve part a of Prob. 3.78, assuming that two 15-N vertical forces have been added, one acting upward at A and the other downward at C. 40 25 N 25 N A 40% B 0.75 m 0.3 m 20 N D 0.4 m E 20 N Fig. P3.78 C X