Question 1131617
      HD     LD   Total
HS  151    11      162
LS    18    20      38
Total 169  31   200

Both low has probability 20/200 or 0.10, the lower right in the table.
HS LD is 11/200 or 0.055 probability 
HS has probability 0.81
HD has probability 0.845
If a specimen has high strength, it certainly can have high density.  They aren't mutually exclusive.  Nor are they independent, since the joint probability is not the same as the actual probability.