Following is a discussion on the Reviewer item titled: Solution to Problem 123 Shear Stress. Feel free to add your own comments!
For the Normal and Shear forces, just have a summation on the vert and hor axis. As for the area (shear area), you have to get the length (cos 20=100/L). Shear area= 50(100/cos20). Then subst to the compressive & shearing stress formulas. The smallest value of P is the safest.