Very simple:
- Parents = 50%
- Grandparents = 25%
- Great-Grandparents = 12.5%
- Great-Great-Grandparents = 6.25%
If your dog has 31.25% of "X" then X must be both a grand parent and a great-great-grandparent in the pedigree (25% + 6.25% = 31.25%) ... or some similar spread.
Hope this clarifies,
Jack