The combined weight of Ryan's dog and Mark's dog is represented by the expression 60 + p pounds.
To determine the combined weight of Ryan's dog and Mark's dog, we need to add the weights of the two dogs together.
Since Ryan's dog weighs 60 pounds and Mark's dog weighs p pounds, the expression to show their combined weight would be 60 + p pounds.
This expression simply adds the known weight of Ryan's dog to the variable representing Mark's dog's weight.
The correct unit for this measurement is pounds because it is the unit given and it is appropriate for the size of the objects, as opposed to ounces which are too small, or tons which are too large for individual dogs.