A sand with c 0 and phi 30 is on the limit of failure the

Question

1. In a sample of sand (c=0) a stress state appears to be possible with σ_xx = 10 kPa, σ_yy = 20 kPa and σ_xy = 5 kPa, without any sign of failure. What can you say of the friction angle φ?

2. A sand, with c = 0 and φ = 30? is on the limit of failure. The minor principal stress is 10 kPa. What is the major principal stress?