Answer
$ \theta = cos ^{-1} (580/700)=34^{\circ}$
Work Step by Step
They can cross the bridge safely. We know that the force, 700 N, times the cosine of the incline angle of the bridge gives the normal force. Thus, we see:
$ F_{max}= Ncos( \theta)$
$ \theta = cos ^{-1} (F_{max}/N)$
This gives:
$ \theta = cos ^{-1} (580/700)=34^{\circ}$
